Add m-esr52 at 52.6.0

1 files changed, 4804 insertions, 0 deletions

diff --git a/layout/generic/nsFlexContainerFrame.cpp b/layout/generic/nsFlexContainerFrame.cpp
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+++ b/layout/generic/nsFlexContainerFrame.cpp
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+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=2 et sw=2 tw=80: */
+
+/* This Source Code is subject to the terms of the Mozilla Public License
+ * version 2.0 (the "License"). You can obtain a copy of the License at
+ * http://mozilla.org/MPL/2.0/. */
+
+/* rendering object for CSS "display: flex" */
+
+#include "mozilla/UniquePtr.h"
+#include "nsFlexContainerFrame.h"
+#include "nsContentUtils.h"
+#include "nsCSSAnonBoxes.h"
+#include "nsDisplayList.h"
+#include "nsIFrameInlines.h"
+#include "nsLayoutUtils.h"
+#include "nsPlaceholderFrame.h"
+#include "nsPresContext.h"
+#include "nsRenderingContext.h"
+#include "nsStyleContext.h"
+#include "mozilla/Logging.h"
+#include <algorithm>
+#include "mozilla/LinkedList.h"
+#include "mozilla/FloatingPoint.h"
+#include "WritingModes.h"
+
+using namespace mozilla;
+using namespace mozilla::layout;
+
+// Convenience typedefs for helper classes that we forward-declare in .h file
+// (so that nsFlexContainerFrame methods can use them as parameters):
+typedef nsFlexContainerFrame::FlexItem FlexItem;
+typedef nsFlexContainerFrame::FlexLine FlexLine;
+typedef nsFlexContainerFrame::FlexboxAxisTracker FlexboxAxisTracker;
+typedef nsFlexContainerFrame::StrutInfo StrutInfo;
+
+static mozilla::LazyLogModule gFlexContainerLog("nsFlexContainerFrame");
+
+// XXXdholbert Some of this helper-stuff should be separated out into a general
+// "main/cross-axis utils" header, shared by grid & flexbox?
+// (Particularly when grid gets support for align-*/justify-* properties.)
+
+// Helper enums
+// ============
+
+// Represents a physical orientation for an axis.
+// The directional suffix indicates the direction in which the axis *grows*.
+// So e.g. eAxis_LR means a horizontal left-to-right axis, whereas eAxis_BT
+// means a vertical bottom-to-top axis.
+// NOTE: The order here is important -- these values are used as indices into
+// the static array 'kAxisOrientationToSidesMap', defined below.
+enum AxisOrientationType {
+  eAxis_LR,
+  eAxis_RL,
+  eAxis_TB,
+  eAxis_BT,
+  eNumAxisOrientationTypes // For sizing arrays that use these values as indices
+};
+
+// Represents one or the other extreme of an axis (e.g. for the main axis, the
+// main-start vs. main-end edge.
+// NOTE: The order here is important -- these values are used as indices into
+// the sub-arrays in 'kAxisOrientationToSidesMap', defined below.
+enum AxisEdgeType {
+  eAxisEdge_Start,
+  eAxisEdge_End,
+  eNumAxisEdges // For sizing arrays that use these values as indices
+};
+
+// This array maps each axis orientation to a pair of corresponding
+// [start, end] physical mozilla::Side values.
+static const mozilla::Side
+kAxisOrientationToSidesMap[eNumAxisOrientationTypes][eNumAxisEdges] = {
+  { eSideLeft,   eSideRight  },  // eAxis_LR
+  { eSideRight,  eSideLeft   },  // eAxis_RL
+  { eSideTop,    eSideBottom },  // eAxis_TB
+  { eSideBottom, eSideTop }      // eAxis_BT
+};
+
+// Helper structs / classes / methods
+// ==================================
+// Returns true iff the given nsStyleDisplay has display:-webkit-{inline-}-box.
+static inline bool
+IsDisplayValueLegacyBox(const nsStyleDisplay* aStyleDisp)
+{
+  return aStyleDisp->mDisplay == mozilla::StyleDisplay::WebkitBox ||
+    aStyleDisp->mDisplay == mozilla::StyleDisplay::WebkitInlineBox;
+}
+
+// Returns true if aFlexContainer is the frame for an element with
+// "display:-webkit-box" or "display:-webkit-inline-box". aFlexContainer is
+// expected to be an instance of nsFlexContainerFrame (enforced with an assert);
+// otherwise, this function's state-bit-check here is bogus.
+static bool
+IsLegacyBox(const nsIFrame* aFlexContainer)
+{
+  MOZ_ASSERT(aFlexContainer->GetType() == nsGkAtoms::flexContainerFrame,
+             "only flex containers may be passed to this function");
+  return aFlexContainer->HasAnyStateBits(NS_STATE_FLEX_IS_LEGACY_WEBKIT_BOX);
+}
+
+// Returns the "align-items" value that's equivalent to the legacy "box-align"
+// value in the given style struct.
+static uint8_t
+ConvertLegacyStyleToAlignItems(const nsStyleXUL* aStyleXUL)
+{
+  // -[moz|webkit]-box-align corresponds to modern "align-items"
+  switch (aStyleXUL->mBoxAlign) {
+    case StyleBoxAlign::Stretch:
+      return NS_STYLE_ALIGN_STRETCH;
+    case StyleBoxAlign::Start:
+      return NS_STYLE_ALIGN_FLEX_START;
+    case StyleBoxAlign::Center:
+      return NS_STYLE_ALIGN_CENTER;
+    case StyleBoxAlign::Baseline:
+      return NS_STYLE_ALIGN_BASELINE;
+    case StyleBoxAlign::End:
+      return NS_STYLE_ALIGN_FLEX_END;
+  }
+
+  MOZ_ASSERT_UNREACHABLE("Unrecognized mBoxAlign enum value");
+  // Fall back to default value of "align-items" property:
+  return NS_STYLE_ALIGN_STRETCH;
+}
+
+// Returns the "justify-content" value that's equivalent to the legacy
+// "box-pack" value in the given style struct.
+static uint8_t
+ConvertLegacyStyleToJustifyContent(const nsStyleXUL* aStyleXUL)
+{
+  // -[moz|webkit]-box-pack corresponds to modern "justify-content"
+  switch (aStyleXUL->mBoxPack) {
+    case StyleBoxPack::Start:
+      return NS_STYLE_ALIGN_FLEX_START;
+    case StyleBoxPack::Center:
+      return NS_STYLE_ALIGN_CENTER;
+    case StyleBoxPack::End:
+      return NS_STYLE_ALIGN_FLEX_END;
+    case StyleBoxPack::Justify:
+      return NS_STYLE_ALIGN_SPACE_BETWEEN;
+  }
+
+  MOZ_ASSERT_UNREACHABLE("Unrecognized mBoxPack enum value");
+  // Fall back to default value of "justify-content" property:
+  return NS_STYLE_ALIGN_FLEX_START;
+}
+
+// Indicates whether advancing along the given axis is equivalent to
+// increasing our X or Y position (as opposed to decreasing it).
+static inline bool
+AxisGrowsInPositiveDirection(AxisOrientationType aAxis)
+{
+  return eAxis_LR == aAxis || eAxis_TB == aAxis;
+}
+
+// Given an AxisOrientationType, returns the "reverse" AxisOrientationType
+// (in the same dimension, but the opposite direction)
+static inline AxisOrientationType
+GetReverseAxis(AxisOrientationType aAxis)
+{
+  AxisOrientationType reversedAxis;
+
+  if (aAxis % 2 == 0) {
+    // even enum value. Add 1 to reverse.
+    reversedAxis = AxisOrientationType(aAxis + 1);
+  } else {
+    // odd enum value. Subtract 1 to reverse.
+    reversedAxis = AxisOrientationType(aAxis - 1);
+  }
+
+  // Check that we're still in the enum's valid range
+  MOZ_ASSERT(reversedAxis >= eAxis_LR &&
+             reversedAxis <= eAxis_BT);
+
+  return reversedAxis;
+}
+
+/**
+ * Converts a "flex-relative" coordinate in a single axis (a main- or cross-axis
+ * coordinate) into a coordinate in the corresponding physical (x or y) axis. If
+ * the flex-relative axis in question already maps *directly* to a physical
+ * axis (i.e. if it's LTR or TTB), then the physical coordinate has the same
+ * numeric value as the provided flex-relative coordinate. Otherwise, we have to
+ * subtract the flex-relative coordinate from the flex container's size in that
+ * axis, to flip the polarity. (So e.g. a main-axis position of 2px in a RTL
+ * 20px-wide container would correspond to a physical coordinate (x-value) of
+ * 18px.)
+ */
+static nscoord
+PhysicalCoordFromFlexRelativeCoord(nscoord aFlexRelativeCoord,
+                                   nscoord aContainerSize,
+                                   AxisOrientationType aAxis) {
+  if (AxisGrowsInPositiveDirection(aAxis)) {
+    return aFlexRelativeCoord;
+  }
+  return aContainerSize - aFlexRelativeCoord;
+}
+
+// Helper-macro to let us pick one of two expressions to evaluate
+// (a width expression vs. a height expression), to get a main-axis or
+// cross-axis component.
+// For code that has e.g. a nsSize object, FlexboxAxisTracker::GetMainComponent
+// and GetCrossComponent are cleaner; but in cases where we simply have
+// two separate expressions for width and height (which may be expensive to
+// evaluate), these macros will ensure that only the expression for the correct
+// axis gets evaluated.
+#define GET_MAIN_COMPONENT(axisTracker_, width_, height_)  \
+  (axisTracker_).IsMainAxisHorizontal() ? (width_) : (height_)
+
+#define GET_CROSS_COMPONENT(axisTracker_, width_, height_)  \
+  (axisTracker_).IsCrossAxisHorizontal() ? (width_) : (height_)
+
+// Logical versions of helper-macros above:
+#define GET_MAIN_COMPONENT_LOGICAL(axisTracker_, wm_, isize_, bsize_)  \
+  wm_.IsOrthogonalTo(axisTracker_.GetWritingMode()) != \
+    (axisTracker_).IsRowOriented() ? (isize_) : (bsize_)
+
+#define GET_CROSS_COMPONENT_LOGICAL(axisTracker_, wm_, isize_, bsize_)  \
+  wm_.IsOrthogonalTo(axisTracker_.GetWritingMode()) != \
+    (axisTracker_).IsRowOriented() ? (bsize_) : (isize_)
+
+// Flags to customize behavior of the FlexboxAxisTracker constructor:
+enum AxisTrackerFlags {
+  eNoFlags = 0x0,
+
+  // Normally, FlexboxAxisTracker may attempt to reverse axes & iteration order
+  // to avoid bottom-to-top child ordering, for saner pagination. This flag
+  // suppresses that behavior (so that we allow bottom-to-top child ordering).
+  // (This may be helpful e.g. when we're only dealing with a single child.)
+  eAllowBottomToTopChildOrdering = 0x1
+};
+MOZ_MAKE_ENUM_CLASS_BITWISE_OPERATORS(AxisTrackerFlags)
+
+// Encapsulates our flex container's main & cross axes.
+class MOZ_STACK_CLASS nsFlexContainerFrame::FlexboxAxisTracker {
+public:
+  FlexboxAxisTracker(const nsFlexContainerFrame* aFlexContainer,
+                     const WritingMode& aWM,
+                     AxisTrackerFlags aFlags = eNoFlags);
+
+  // Accessors:
+  // XXXdholbert [BEGIN DEPRECATED]
+  AxisOrientationType GetMainAxis() const  { return mMainAxis;  }
+  AxisOrientationType GetCrossAxis() const { return mCrossAxis; }
+
+  bool IsMainAxisHorizontal() const {
+    // If we're row-oriented, and our writing mode is NOT vertical,
+    // or we're column-oriented and our writing mode IS vertical,
+    // then our main axis is horizontal. This handles all cases:
+    return mIsRowOriented != mWM.IsVertical();
+  }
+  bool IsCrossAxisHorizontal() const {
+    return !IsMainAxisHorizontal();
+  }
+  // XXXdholbert [END DEPRECATED]
+
+  // Returns the flex container's writing mode.
+  WritingMode GetWritingMode() const { return mWM; }
+
+  // Returns true if our main axis is in the reverse direction of our
+  // writing mode's corresponding axis. (From 'flex-direction: *-reverse')
+  bool IsMainAxisReversed() const {
+    return mIsMainAxisReversed;
+  }
+  // Returns true if our cross axis is in the reverse direction of our
+  // writing mode's corresponding axis. (From 'flex-wrap: *-reverse')
+  bool IsCrossAxisReversed() const {
+    return mIsCrossAxisReversed;
+  }
+
+  bool IsRowOriented() const { return mIsRowOriented; }
+  bool IsColumnOriented() const { return !mIsRowOriented; }
+
+  nscoord GetMainComponent(const nsSize& aSize) const {
+    return GET_MAIN_COMPONENT(*this, aSize.width, aSize.height);
+  }
+  int32_t GetMainComponent(const LayoutDeviceIntSize& aIntSize) const {
+    return GET_MAIN_COMPONENT(*this, aIntSize.width, aIntSize.height);
+  }
+
+  nscoord GetCrossComponent(const nsSize& aSize) const {
+    return GET_CROSS_COMPONENT(*this, aSize.width, aSize.height);
+  }
+  int32_t GetCrossComponent(const LayoutDeviceIntSize& aIntSize) const {
+    return GET_CROSS_COMPONENT(*this, aIntSize.width, aIntSize.height);
+  }
+
+  nscoord GetMarginSizeInMainAxis(const nsMargin& aMargin) const {
+    return IsMainAxisHorizontal() ?
+      aMargin.LeftRight() :
+      aMargin.TopBottom();
+  }
+  nscoord GetMarginSizeInCrossAxis(const nsMargin& aMargin) const {
+    return IsCrossAxisHorizontal() ?
+      aMargin.LeftRight() :
+      aMargin.TopBottom();
+  }
+
+  // Returns aFrame's computed value for 'height' or 'width' -- whichever is in
+  // the cross-axis. (NOTE: This is cross-axis-specific for now. If we need a
+  // main-axis version as well, we could generalize or clone this function.)
+  const nsStyleCoord& ComputedCrossSize(const nsIFrame* aFrame) const {
+    const nsStylePosition* stylePos = aFrame->StylePosition();
+
+    return IsCrossAxisHorizontal() ?
+      stylePos->mWidth :
+      stylePos->mHeight;
+  }
+
+  /**
+   * Converts a "flex-relative" point (a main-axis & cross-axis coordinate)
+   * into a LogicalPoint, using the flex container's writing mode.
+   *
+   *  @arg aMainCoord  The main-axis coordinate -- i.e an offset from the
+   *                   main-start edge of the flex container's content box.
+   *  @arg aCrossCoord The cross-axis coordinate -- i.e an offset from the
+   *                   cross-start edge of the flex container's content box.
+   *  @arg aContainerMainSize  The main size of flex container's content box.
+   *  @arg aContainerCrossSize The cross size of flex container's content box.
+   *  @return A LogicalPoint, with the flex container's writing mode, that
+   *          represents the same position. The logical coordinates are
+   *          relative to the flex container's content box.
+   */
+  LogicalPoint
+  LogicalPointFromFlexRelativePoint(nscoord aMainCoord,
+                                    nscoord aCrossCoord,
+                                    nscoord aContainerMainSize,
+                                    nscoord aContainerCrossSize) const {
+    nscoord logicalCoordInMainAxis = mIsMainAxisReversed ?
+      aContainerMainSize - aMainCoord : aMainCoord;
+    nscoord logicalCoordInCrossAxis = mIsCrossAxisReversed ?
+      aContainerCrossSize - aCrossCoord : aCrossCoord;
+
+    return mIsRowOriented ?
+      LogicalPoint(mWM, logicalCoordInMainAxis, logicalCoordInCrossAxis) :
+      LogicalPoint(mWM, logicalCoordInCrossAxis, logicalCoordInMainAxis);
+  }
+
+  /**
+   * Converts a "flex-relative" size (a main-axis & cross-axis size)
+   * into a LogicalSize, using the flex container's writing mode.
+   *
+   *  @arg aMainSize  The main-axis size.
+   *  @arg aCrossSize The cross-axis size.
+   *  @return A LogicalSize, with the flex container's writing mode, that
+   *          represents the same size.
+   */
+  LogicalSize LogicalSizeFromFlexRelativeSizes(nscoord aMainSize,
+                                               nscoord aCrossSize) const {
+    return mIsRowOriented ?
+      LogicalSize(mWM, aMainSize, aCrossSize) :
+      LogicalSize(mWM, aCrossSize, aMainSize);
+  }
+
+  // Are my axes reversed with respect to what the author asked for?
+  // (We may reverse the axes in the FlexboxAxisTracker constructor and set
+  // this flag, to avoid reflowing our children in bottom-to-top order.)
+  bool AreAxesInternallyReversed() const
+  {
+    return mAreAxesInternallyReversed;
+  }
+
+private:
+  // Delete copy-constructor & reassignment operator, to prevent accidental
+  // (unnecessary) copying.
+  FlexboxAxisTracker(const FlexboxAxisTracker&) = delete;
+  FlexboxAxisTracker& operator=(const FlexboxAxisTracker&) = delete;
+
+  // Helpers for constructor which determine the orientation of our axes, based
+  // on legacy box properties (-webkit-box-orient, -webkit-box-direction) or
+  // modern flexbox properties (flex-direction, flex-wrap) depending on whether
+  // the flex container is a "legacy box" (as determined by IsLegacyBox).
+  void InitAxesFromLegacyProps(const nsFlexContainerFrame* aFlexContainer);
+  void InitAxesFromModernProps(const nsFlexContainerFrame* aFlexContainer);
+
+  // XXXdholbert [BEGIN DEPRECATED]
+  AxisOrientationType mMainAxis;
+  AxisOrientationType mCrossAxis;
+  // XXXdholbert [END DEPRECATED]
+
+  const WritingMode mWM; // The flex container's writing mode.
+
+  bool mIsRowOriented; // Is our main axis the inline axis?
+                       // (Are we 'flex-direction:row[-reverse]'?)
+
+  bool mIsMainAxisReversed; // Is our main axis in the opposite direction
+                            // as mWM's corresponding axis? (e.g. RTL vs LTR)
+  bool mIsCrossAxisReversed; // Is our cross axis in the opposite direction
+                             // as mWM's corresponding axis? (e.g. BTT vs TTB)
+
+  // Implementation detail -- this indicates whether we've decided to
+  // transparently reverse our axes & our child ordering, to avoid having
+  // frames flow from bottom to top in either axis (& to make pagination saner).
+  bool mAreAxesInternallyReversed;
+};
+
+/**
+ * Represents a flex item.
+ * Includes the various pieces of input that the Flexbox Layout Algorithm uses
+ * to resolve a flexible width.
+ */
+class nsFlexContainerFrame::FlexItem : public LinkedListElement<FlexItem>
+{
+public:
+  // Normal constructor:
+  FlexItem(ReflowInput& aFlexItemReflowInput,
+           float aFlexGrow, float aFlexShrink, nscoord aMainBaseSize,
+           nscoord aMainMinSize, nscoord aMainMaxSize,
+           nscoord aTentativeCrossSize,
+           nscoord aCrossMinSize, nscoord aCrossMaxSize,
+           const FlexboxAxisTracker& aAxisTracker);
+
+  // Simplified constructor, to be used only for generating "struts":
+  // (NOTE: This "strut" constructor uses the *container's* writing mode, which
+  // we'll use on this FlexItem instead of the child frame's real writing mode.
+  // This is fine - it doesn't matter what writing mode we use for a
+  // strut, since it won't render any content and we already know its size.)
+  FlexItem(nsIFrame* aChildFrame, nscoord aCrossSize, WritingMode aContainerWM);
+
+  // Accessors
+  nsIFrame* Frame() const          { return mFrame; }
+  nscoord GetFlexBaseSize() const  { return mFlexBaseSize; }
+
+  nscoord GetMainMinSize() const   {
+    MOZ_ASSERT(!mNeedsMinSizeAutoResolution,
+               "Someone's using an unresolved 'auto' main min-size");
+    return mMainMinSize;
+  }
+  nscoord GetMainMaxSize() const   { return mMainMaxSize; }
+
+  // Note: These return the main-axis position and size of our *content box*.
+  nscoord GetMainSize() const      { return mMainSize; }
+  nscoord GetMainPosition() const  { return mMainPosn; }
+
+  nscoord GetCrossMinSize() const  { return mCrossMinSize; }
+  nscoord GetCrossMaxSize() const  { return mCrossMaxSize; }
+
+  // Note: These return the cross-axis position and size of our *content box*.
+  nscoord GetCrossSize() const     { return mCrossSize;  }
+  nscoord GetCrossPosition() const { return mCrossPosn; }
+
+  nscoord ResolvedAscent(bool aUseFirstBaseline) const {
+    if (mAscent == ReflowOutput::ASK_FOR_BASELINE) {
+      // XXXdholbert We should probably be using the *container's* writing-mode
+      // here, instead of the item's -- though it doesn't much matter right
+      // now, because all of the baseline-handling code here essentially
+      // assumes that the container & items have the same writing-mode. This
+      // will matter more (& can be expanded/tested) once we officially support
+      // logical directions & vertical writing-modes in flexbox, in bug 1079155
+      // or a dependency.
+      // Use GetFirstLineBaseline() or GetLastLineBaseline() as appropriate,
+      // or just GetLogicalBaseline() if that fails.
+      bool found = aUseFirstBaseline ?
+        nsLayoutUtils::GetFirstLineBaseline(mWM, mFrame, &mAscent) :
+        nsLayoutUtils::GetLastLineBaseline(mWM, mFrame, &mAscent);
+
+      if (!found) {
+        mAscent = mFrame->SynthesizeBaselineBOffsetFromBorderBox(mWM,
+                            BaselineSharingGroup::eFirst);
+      }
+    }
+    return mAscent;
+  }
+
+  // Convenience methods to compute the main & cross size of our *margin-box*.
+  // The caller is responsible for telling us the right axis, so that we can
+  // pull out the appropriate components of our margin/border/padding structs.
+  nscoord GetOuterMainSize(AxisOrientationType aMainAxis) const
+  {
+    return mMainSize + GetMarginBorderPaddingSizeInAxis(aMainAxis);
+  }
+
+  nscoord GetOuterCrossSize(AxisOrientationType aCrossAxis) const
+  {
+    return mCrossSize + GetMarginBorderPaddingSizeInAxis(aCrossAxis);
+  }
+
+  // Returns the distance between this FlexItem's baseline and the cross-start
+  // edge of its margin-box. Used in baseline alignment.
+  // (This function needs to be told which edge we're measuring the baseline
+  // from, so that it can look up the appropriate components from mMargin.)
+  nscoord GetBaselineOffsetFromOuterCrossEdge(
+    AxisEdgeType aEdge,
+    const FlexboxAxisTracker& aAxisTracker,
+    bool aUseFirstLineBaseline) const;
+
+  float GetShareOfWeightSoFar() const { return mShareOfWeightSoFar; }
+
+  bool IsFrozen() const            { return mIsFrozen; }
+
+  bool HadMinViolation() const     { return mHadMinViolation; }
+  bool HadMaxViolation() const     { return mHadMaxViolation; }
+
+  // Indicates whether this item received a preliminary "measuring" reflow
+  // before its actual reflow.
+  bool HadMeasuringReflow() const  { return mHadMeasuringReflow; }
+
+  // Indicates whether this item's cross-size has been stretched (from having
+  // "align-self: stretch" with an auto cross-size and no auto margins in the
+  // cross axis).
+  bool IsStretched() const         { return mIsStretched; }
+
+  // Indicates whether we need to resolve an 'auto' value for the main-axis
+  // min-[width|height] property.
+  bool NeedsMinSizeAutoResolution() const
+    { return mNeedsMinSizeAutoResolution; }
+
+  // Indicates whether this item is a "strut" left behind by an element with
+  // visibility:collapse.
+  bool IsStrut() const             { return mIsStrut; }
+
+  WritingMode GetWritingMode() const { return mWM; }
+  uint8_t GetAlignSelf() const     { return mAlignSelf; }
+
+  // Returns the flex factor (flex-grow or flex-shrink), depending on
+  // 'aIsUsingFlexGrow'.
+  //
+  // Asserts fatally if called on a frozen item (since frozen items are not
+  // flexible).
+  float GetFlexFactor(bool aIsUsingFlexGrow)
+  {
+    MOZ_ASSERT(!IsFrozen(), "shouldn't need flex factor after item is frozen");
+
+    return aIsUsingFlexGrow ? mFlexGrow : mFlexShrink;
+  }
+
+  // Returns the weight that we should use in the "resolving flexible lengths"
+  // algorithm.  If we're using the flex grow factor, we just return that;
+  // otherwise, we return the "scaled flex shrink factor" (scaled by our flex
+  // base size, so that when both large and small items are shrinking, the large
+  // items shrink more).
+  //
+  // I'm calling this a "weight" instead of a "[scaled] flex-[grow|shrink]
+  // factor", to more clearly distinguish it from the actual flex-grow &
+  // flex-shrink factors.
+  //
+  // Asserts fatally if called on a frozen item (since frozen items are not
+  // flexible).
+  float GetWeight(bool aIsUsingFlexGrow)
+  {
+    MOZ_ASSERT(!IsFrozen(), "shouldn't need weight after item is frozen");
+
+    if (aIsUsingFlexGrow) {
+      return mFlexGrow;
+    }
+
+    // We're using flex-shrink --> return mFlexShrink * mFlexBaseSize
+    if (mFlexBaseSize == 0) {
+      // Special-case for mFlexBaseSize == 0 -- we have no room to shrink, so
+      // regardless of mFlexShrink, we should just return 0.
+      // (This is really a special-case for when mFlexShrink is infinity, to
+      // avoid performing mFlexShrink * mFlexBaseSize = inf * 0 = undefined.)
+      return 0.0f;
+    }
+    return mFlexShrink * mFlexBaseSize;
+  }
+
+  const nsSize& IntrinsicRatio() const { return mIntrinsicRatio; }
+  bool HasIntrinsicRatio() const { return mIntrinsicRatio != nsSize(); }
+
+  // Getters for margin:
+  // ===================
+  const nsMargin& GetMargin() const { return mMargin; }
+
+  // Returns the margin component for a given mozilla::Side
+  nscoord GetMarginComponentForSide(mozilla::Side aSide) const
+  { return mMargin.Side(aSide); }
+
+  // Returns the total space occupied by this item's margins in the given axis
+  nscoord GetMarginSizeInAxis(AxisOrientationType aAxis) const
+  {
+    mozilla::Side startSide = kAxisOrientationToSidesMap[aAxis][eAxisEdge_Start];
+    mozilla::Side endSide = kAxisOrientationToSidesMap[aAxis][eAxisEdge_End];
+    return GetMarginComponentForSide(startSide) +
+      GetMarginComponentForSide(endSide);
+  }
+
+  // Getters for border/padding
+  // ==========================
+  const nsMargin& GetBorderPadding() const { return mBorderPadding; }
+
+  // Returns the border+padding component for a given mozilla::Side
+  nscoord GetBorderPaddingComponentForSide(mozilla::Side aSide) const
+  { return mBorderPadding.Side(aSide); }
+
+  // Returns the total space occupied by this item's borders and padding in
+  // the given axis
+  nscoord GetBorderPaddingSizeInAxis(AxisOrientationType aAxis) const
+  {
+    mozilla::Side startSide = kAxisOrientationToSidesMap[aAxis][eAxisEdge_Start];
+    mozilla::Side endSide = kAxisOrientationToSidesMap[aAxis][eAxisEdge_End];
+    return GetBorderPaddingComponentForSide(startSide) +
+      GetBorderPaddingComponentForSide(endSide);
+  }
+
+  // Getter for combined margin/border/padding
+  // =========================================
+  // Returns the total space occupied by this item's margins, borders and
+  // padding in the given axis
+  nscoord GetMarginBorderPaddingSizeInAxis(AxisOrientationType aAxis) const
+  {
+    return GetMarginSizeInAxis(aAxis) + GetBorderPaddingSizeInAxis(aAxis);
+  }
+
+  // Setters
+  // =======
+  // Helper to set the resolved value of min-[width|height]:auto for the main
+  // axis. (Should only be used if NeedsMinSizeAutoResolution() returns true.)
+  void UpdateMainMinSize(nscoord aNewMinSize)
+  {
+    NS_ASSERTION(aNewMinSize >= 0,
+                 "How did we end up with a negative min-size?");
+    MOZ_ASSERT(mMainMaxSize >= aNewMinSize,
+               "Should only use this function for resolving min-size:auto, "
+               "and main max-size should be an upper-bound for resolved val");
+    MOZ_ASSERT(mNeedsMinSizeAutoResolution &&
+               (mMainMinSize == 0 || mFrame->IsThemed(mFrame->StyleDisplay())),
+               "Should only use this function for resolving min-size:auto, "
+               "so we shouldn't already have a nonzero min-size established "
+               "(unless it's a themed-widget-imposed minimum size)");
+
+    if (aNewMinSize > mMainMinSize) {
+      mMainMinSize = aNewMinSize;
+      // Also clamp main-size to be >= new min-size:
+      mMainSize = std::max(mMainSize, aNewMinSize);
+    }
+    mNeedsMinSizeAutoResolution = false;
+  }
+
+  // This sets our flex base size, and then sets our main size to the
+  // resulting "hypothetical main size" (the base size clamped to our
+  // main-axis [min,max] sizing constraints).
+  void SetFlexBaseSizeAndMainSize(nscoord aNewFlexBaseSize)
+  {
+    MOZ_ASSERT(!mIsFrozen || mFlexBaseSize == NS_INTRINSICSIZE,
+               "flex base size shouldn't change after we're frozen "
+               "(unless we're just resolving an intrinsic size)");
+    mFlexBaseSize = aNewFlexBaseSize;
+
+    // Before we've resolved flexible lengths, we keep mMainSize set to
+    // the 'hypothetical main size', which is the flex base size, clamped
+    // to the [min,max] range:
+    mMainSize = NS_CSS_MINMAX(mFlexBaseSize, mMainMinSize, mMainMaxSize);
+  }
+
+  // Setters used while we're resolving flexible lengths
+  // ---------------------------------------------------
+
+  // Sets the main-size of our flex item's content-box.
+  void SetMainSize(nscoord aNewMainSize)
+  {
+    MOZ_ASSERT(!mIsFrozen, "main size shouldn't change after we're frozen");
+    mMainSize = aNewMainSize;
+  }
+
+  void SetShareOfWeightSoFar(float aNewShare)
+  {
+    MOZ_ASSERT(!mIsFrozen || aNewShare == 0.0f,
+               "shouldn't be giving this item any share of the weight "
+               "after it's frozen");
+    mShareOfWeightSoFar = aNewShare;
+  }
+
+  void Freeze() { mIsFrozen = true; }
+
+  void SetHadMinViolation()
+  {
+    MOZ_ASSERT(!mIsFrozen,
+               "shouldn't be changing main size & having violations "
+               "after we're frozen");
+    mHadMinViolation = true;
+  }
+  void SetHadMaxViolation()
+  {
+    MOZ_ASSERT(!mIsFrozen,
+               "shouldn't be changing main size & having violations "
+               "after we're frozen");
+    mHadMaxViolation = true;
+  }
+  void ClearViolationFlags()
+  { mHadMinViolation = mHadMaxViolation = false; }
+
+  // Setters for values that are determined after we've resolved our main size
+  // -------------------------------------------------------------------------
+
+  // Sets the main-axis position of our flex item's content-box.
+  // (This is the distance between the main-start edge of the flex container
+  // and the main-start edge of the flex item's content-box.)
+  void SetMainPosition(nscoord aPosn) {
+    MOZ_ASSERT(mIsFrozen, "main size should be resolved before this");
+    mMainPosn  = aPosn;
+  }
+
+  // Sets the cross-size of our flex item's content-box.
+  void SetCrossSize(nscoord aCrossSize) {
+    MOZ_ASSERT(!mIsStretched,
+               "Cross size shouldn't be modified after it's been stretched");
+    mCrossSize = aCrossSize;
+  }
+
+  // Sets the cross-axis position of our flex item's content-box.
+  // (This is the distance between the cross-start edge of the flex container
+  // and the cross-start edge of the flex item.)
+  void SetCrossPosition(nscoord aPosn) {
+    MOZ_ASSERT(mIsFrozen, "main size should be resolved before this");
+    mCrossPosn = aPosn;
+  }
+
+  // After a FlexItem has had a reflow, this method can be used to cache its
+  // (possibly-unresolved) ascent, in case it's needed later for
+  // baseline-alignment or to establish the container's baseline.
+  // (NOTE: This can be marked 'const' even though it's modifying mAscent,
+  // because mAscent is mutable. It's nice for this to be 'const', because it
+  // means our final reflow can iterate over const FlexItem pointers, and we
+  // can be sure it's not modifying those FlexItems, except via this method.)
+  void SetAscent(nscoord aAscent) const {
+    mAscent = aAscent; // NOTE: this may be ASK_FOR_BASELINE
+  }
+
+  void SetHadMeasuringReflow() {
+    mHadMeasuringReflow = true;
+  }
+
+  void SetIsStretched() {
+    MOZ_ASSERT(mIsFrozen, "main size should be resolved before this");
+    mIsStretched = true;
+  }
+
+  // Setter for margin components (for resolving "auto" margins)
+  void SetMarginComponentForSide(mozilla::Side aSide, nscoord aLength)
+  {
+    MOZ_ASSERT(mIsFrozen, "main size should be resolved before this");
+    mMargin.Side(aSide) = aLength;
+  }
+
+  void ResolveStretchedCrossSize(nscoord aLineCrossSize,
+                                 const FlexboxAxisTracker& aAxisTracker);
+
+  uint32_t GetNumAutoMarginsInAxis(AxisOrientationType aAxis) const;
+
+  // Once the main size has been resolved, should we bother doing layout to
+  // establish the cross size?
+  bool CanMainSizeInfluenceCrossSize(const FlexboxAxisTracker& aAxisTracker) const;
+
+protected:
+  // Helper called by the constructor, to set mNeedsMinSizeAutoResolution:
+  void CheckForMinSizeAuto(const ReflowInput& aFlexItemReflowInput,
+                           const FlexboxAxisTracker& aAxisTracker);
+
+  // Our frame:
+  nsIFrame* const mFrame;
+
+  // Values that we already know in constructor: (and are hence mostly 'const')
+  const float mFlexGrow;
+  const float mFlexShrink;
+
+  const nsSize mIntrinsicRatio;
+
+  const nsMargin mBorderPadding;
+  nsMargin mMargin; // non-const because we need to resolve auto margins
+
+  // These are non-const so that we can lazily update them with the item's
+  // intrinsic size (obtained via a "measuring" reflow), when necessary.
+  // (e.g. for "flex-basis:auto;height:auto" & "min-height:auto")
+  nscoord mFlexBaseSize;
+  nscoord mMainMinSize;
+  nscoord mMainMaxSize;
+
+  const nscoord mCrossMinSize;
+  const nscoord mCrossMaxSize;
+
+  // Values that we compute after constructor:
+  nscoord mMainSize;
+  nscoord mMainPosn;
+  nscoord mCrossSize;
+  nscoord mCrossPosn;
+  mutable nscoord mAscent; // Mutable b/c it's set & resolved lazily, sometimes
+                           // via const pointer. See comment above SetAscent().
+
+  // Temporary state, while we're resolving flexible widths (for our main size)
+  // XXXdholbert To save space, we could use a union to make these variables
+  // overlay the same memory as some other member vars that aren't touched
+  // until after main-size has been resolved. In particular, these could share
+  // memory with mMainPosn through mAscent, and mIsStretched.
+  float mShareOfWeightSoFar;
+  bool mIsFrozen;
+  bool mHadMinViolation;
+  bool mHadMaxViolation;
+
+  // Misc:
+  bool mHadMeasuringReflow; // Did this item get a preliminary reflow,
+                            // to measure its desired height?
+  bool mIsStretched; // See IsStretched() documentation
+  bool mIsStrut;     // Is this item a "strut" left behind by an element
+                     // with visibility:collapse?
+
+  // Does this item need to resolve a min-[width|height]:auto (in main-axis).
+  bool mNeedsMinSizeAutoResolution;
+
+  const WritingMode mWM; // The flex item's writing mode.
+  uint8_t mAlignSelf; // My "align-self" computed value (with "auto"
+                      // swapped out for parent"s "align-items" value,
+                      // in our constructor).
+};
+
+/**
+ * Represents a single flex line in a flex container.
+ * Manages a linked list of the FlexItems that are in the line.
+ */
+class nsFlexContainerFrame::FlexLine : public LinkedListElement<FlexLine>
+{
+public:
+  FlexLine()
+  : mNumItems(0),
+    mNumFrozenItems(0),
+    mTotalInnerHypotheticalMainSize(0),
+    mTotalOuterHypotheticalMainSize(0),
+    mLineCrossSize(0),
+    mFirstBaselineOffset(nscoord_MIN),
+    mLastBaselineOffset(nscoord_MIN)
+  {}
+
+  // Returns the sum of our FlexItems' outer hypothetical main sizes.
+  // ("outer" = margin-box, and "hypothetical" = before flexing)
+  nscoord GetTotalOuterHypotheticalMainSize() const {
+    return mTotalOuterHypotheticalMainSize;
+  }
+
+  // Accessors for our FlexItems & information about them:
+  FlexItem* GetFirstItem()
+  {
+    MOZ_ASSERT(mItems.isEmpty() == (mNumItems == 0),
+               "mNumItems bookkeeping is off");
+    return mItems.getFirst();
+  }
+
+  const FlexItem* GetFirstItem() const
+  {
+    MOZ_ASSERT(mItems.isEmpty() == (mNumItems == 0),
+               "mNumItems bookkeeping is off");
+    return mItems.getFirst();
+  }
+
+  FlexItem* GetLastItem()
+  {
+    MOZ_ASSERT(mItems.isEmpty() == (mNumItems == 0),
+               "mNumItems bookkeeping is off");
+    return mItems.getLast();
+  }
+
+  const FlexItem* GetLastItem() const
+  {
+    MOZ_ASSERT(mItems.isEmpty() == (mNumItems == 0),
+               "mNumItems bookkeeping is off");
+    return mItems.getLast();
+  }
+
+  bool IsEmpty() const
+  {
+    MOZ_ASSERT(mItems.isEmpty() == (mNumItems == 0),
+               "mNumItems bookkeeping is off");
+    return mItems.isEmpty();
+  }
+
+  uint32_t NumItems() const
+  {
+    MOZ_ASSERT(mItems.isEmpty() == (mNumItems == 0),
+               "mNumItems bookkeeping is off");
+    return mNumItems;
+  }
+
+  // Adds the given FlexItem to our list of items (at the front or back
+  // depending on aShouldInsertAtFront), and adds its hypothetical
+  // outer & inner main sizes to our totals. Use this method instead of
+  // directly modifying the item list, so that our bookkeeping remains correct.
+  void AddItem(FlexItem* aItem,
+               bool aShouldInsertAtFront,
+               nscoord aItemInnerHypotheticalMainSize,
+               nscoord aItemOuterHypotheticalMainSize)
+  {
+    if (aShouldInsertAtFront) {
+      mItems.insertFront(aItem);
+    } else {
+      mItems.insertBack(aItem);
+    }
+
+    // Update our various bookkeeping member-vars:
+    mNumItems++;
+    if (aItem->IsFrozen()) {
+      mNumFrozenItems++;
+    }
+    mTotalInnerHypotheticalMainSize += aItemInnerHypotheticalMainSize;
+    mTotalOuterHypotheticalMainSize += aItemOuterHypotheticalMainSize;
+  }
+
+  // Computes the cross-size and baseline position of this FlexLine, based on
+  // its FlexItems.
+  void ComputeCrossSizeAndBaseline(const FlexboxAxisTracker& aAxisTracker);
+
+  // Returns the cross-size of this line.
+  nscoord GetLineCrossSize() const { return mLineCrossSize; }
+
+  // Setter for line cross-size -- needed for cases where the flex container
+  // imposes a cross-size on the line. (e.g. for single-line flexbox, or for
+  // multi-line flexbox with 'align-content: stretch')
+  void SetLineCrossSize(nscoord aLineCrossSize) {
+    mLineCrossSize = aLineCrossSize;
+  }
+
+  /**
+   * Returns the offset within this line where any baseline-aligned FlexItems
+   * should place their baseline. Usually, this represents a distance from the
+   * line's cross-start edge, but if we're internally reversing the axes (see
+   * AreAxesInternallyReversed()), this instead represents the distance from
+   * its cross-end edge.
+   *
+   * If there are no baseline-aligned FlexItems, returns nscoord_MIN.
+   */
+  nscoord GetFirstBaselineOffset() const {
+    return mFirstBaselineOffset;
+  }
+
+  /**
+   * Returns the offset within this line where any last baseline-aligned
+   * FlexItems should place their baseline. Opposite the case of the first
+   * baseline offset, this represents a distance from the line's cross-end
+   * edge (since last baseline-aligned items are flush to the cross-end edge).
+   * If we're internally reversing the axes, this instead represents the
+   * distance from the line's cross-start edge.
+   *
+   * If there are no last baseline-aligned FlexItems, returns nscoord_MIN.
+   */
+  nscoord GetLastBaselineOffset() const {
+    return mLastBaselineOffset;
+  }
+
+  // Runs the "Resolving Flexible Lengths" algorithm from section 9.7 of the
+  // CSS flexbox spec to distribute aFlexContainerMainSize among our flex items.
+  void ResolveFlexibleLengths(nscoord aFlexContainerMainSize);
+
+  void PositionItemsInMainAxis(uint8_t aJustifyContent,
+                               nscoord aContentBoxMainSize,
+                               const FlexboxAxisTracker& aAxisTracker);
+
+  void PositionItemsInCrossAxis(nscoord aLineStartPosition,
+                                const FlexboxAxisTracker& aAxisTracker);
+
+  friend class AutoFlexLineListClearer; // (needs access to mItems)
+
+private:
+  // Helpers for ResolveFlexibleLengths():
+  void FreezeItemsEarly(bool aIsUsingFlexGrow);
+
+  void FreezeOrRestoreEachFlexibleSize(const nscoord aTotalViolation,
+                                       bool aIsFinalIteration);
+
+  LinkedList<FlexItem> mItems; // Linked list of this line's flex items.
+
+  uint32_t mNumItems; // Number of FlexItems in this line (in |mItems|).
+                      // (Shouldn't change after GenerateFlexLines finishes
+                      // with this line -- at least, not until we add support
+                      // for splitting lines across continuations. Then we can
+                      // update this count carefully.)
+
+  // Number of *frozen* FlexItems in this line, based on FlexItem::IsFrozen().
+  // Mostly used for optimization purposes, e.g. to bail out early from loops
+  // when we can tell they have nothing left to do.
+  uint32_t mNumFrozenItems;
+
+  nscoord mTotalInnerHypotheticalMainSize;
+  nscoord mTotalOuterHypotheticalMainSize;
+  nscoord mLineCrossSize;
+  nscoord mFirstBaselineOffset;
+  nscoord mLastBaselineOffset;
+};
+
+// Information about a strut left behind by a FlexItem that's been collapsed
+// using "visibility:collapse".
+struct nsFlexContainerFrame::StrutInfo {
+  StrutInfo(uint32_t aItemIdx, nscoord aStrutCrossSize)
+    : mItemIdx(aItemIdx),
+      mStrutCrossSize(aStrutCrossSize)
+  {
+  }
+
+  uint32_t mItemIdx;      // Index in the child list.
+  nscoord mStrutCrossSize; // The cross-size of this strut.
+};
+
+static void
+BuildStrutInfoFromCollapsedItems(const FlexLine* aFirstLine,
+                                 nsTArray<StrutInfo>& aStruts)
+{
+  MOZ_ASSERT(aFirstLine, "null first line pointer");
+  MOZ_ASSERT(aStruts.IsEmpty(),
+             "We should only build up StrutInfo once per reflow, so "
+             "aStruts should be empty when this is called");
+
+  uint32_t itemIdxInContainer = 0;
+  for (const FlexLine* line = aFirstLine; line; line = line->getNext()) {
+    for (const FlexItem* item = line->GetFirstItem(); item;
+         item = item->getNext()) {
+      if (NS_STYLE_VISIBILITY_COLLAPSE ==
+          item->Frame()->StyleVisibility()->mVisible) {
+        // Note the cross size of the line as the item's strut size.
+        aStruts.AppendElement(StrutInfo(itemIdxInContainer,
+                                        line->GetLineCrossSize()));
+      }
+      itemIdxInContainer++;
+    }
+  }
+}
+
+// Convenience function to get either the "order" or the "box-ordinal-group"
+// property-value for a flex item (depending on whether the container is a
+// modern flex container or a legacy box).
+static int32_t
+GetOrderOrBoxOrdinalGroup(nsIFrame* aFlexItem, bool aIsLegacyBox)
+{
+  if (aFlexItem->GetType() == nsGkAtoms::placeholderFrame) {
+    // Always treat placeholders as having the default value, which is
+    // 1 for (legacy) 'box-ordinal-group' and 0 for 'order'.
+    return aIsLegacyBox ? 1 : 0;
+  }
+  if (aIsLegacyBox) {
+    // We'll be using mBoxOrdinal, which has type uint32_t. However, the modern
+    // 'order' property (whose functionality we're co-opting) has type int32_t.
+    // So: if we happen to have a uint32_t value that's greater than INT32_MAX,
+    // we clamp it rather than letting it overflow. Chances are, this is just
+    // an author using BIG_VALUE anyway, so the clamped value should be fine.
+    // (particularly since sufficiently-huge values are busted in Chrome/WebKit
+    // per https://bugs.chromium.org/p/chromium/issues/detail?id=599645 )
+    uint32_t clampedBoxOrdinal = std::min(aFlexItem->StyleXUL()->mBoxOrdinal,
+                                          static_cast<uint32_t>(INT32_MAX));
+    return static_cast<int32_t>(clampedBoxOrdinal);
+  }
+
+  // Normal case: just use modern 'order' property.
+  return aFlexItem->StylePosition()->mOrder;
+}
+
+// Helper-function to find the first non-anonymous-box descendent of aFrame.
+static nsIFrame*
+GetFirstNonAnonBoxDescendant(nsIFrame* aFrame)
+{
+  while (aFrame) {
+    nsIAtom* pseudoTag = aFrame->StyleContext()->GetPseudo();
+
+    // If aFrame isn't an anonymous container, then it'll do.
+    if (!pseudoTag ||                                 // No pseudotag.
+        !nsCSSAnonBoxes::IsAnonBox(pseudoTag) ||      // Pseudotag isn't anon.
+        nsCSSAnonBoxes::IsNonElement(pseudoTag)) {    // Text, not a container.
+      break;
+    }
+
+    // Otherwise, descend to its first child and repeat.
+
+    // SPECIAL CASE: if we're dealing with an anonymous table, then it might
+    // be wrapping something non-anonymous in its caption or col-group lists
+    // (instead of its principal child list), so we have to look there.
+    // (Note: For anonymous tables that have a non-anon cell *and* a non-anon
+    // column, we'll always return the column. This is fine; we're really just
+    // looking for a handle to *anything* with a meaningful content node inside
+    // the table, for use in DOM comparisons to things outside of the table.)
+    if (MOZ_UNLIKELY(aFrame->GetType() == nsGkAtoms::tableWrapperFrame)) {
+      nsIFrame* captionDescendant =
+        GetFirstNonAnonBoxDescendant(aFrame->GetChildList(kCaptionList).FirstChild());
+      if (captionDescendant) {
+        return captionDescendant;
+      }
+    } else if (MOZ_UNLIKELY(aFrame->GetType() == nsGkAtoms::tableFrame)) {
+      nsIFrame* colgroupDescendant =
+        GetFirstNonAnonBoxDescendant(aFrame->GetChildList(kColGroupList).FirstChild());
+      if (colgroupDescendant) {
+        return colgroupDescendant;
+      }
+    }
+
+    // USUAL CASE: Descend to the first child in principal list.
+    aFrame = aFrame->PrincipalChildList().FirstChild();
+  }
+  return aFrame;
+}
+
+/**
+ * Sorting helper-function that compares two frames' "order" property-values,
+ * and if they're equal, compares the DOM positions of their corresponding
+ * content nodes. Returns true if aFrame1 is "less than or equal to" aFrame2
+ * according to this comparison.
+ *
+ * Note: This can't be a static function, because we need to pass it as a
+ * template argument. (Only functions with external linkage can be passed as
+ * template arguments.)
+ *
+ * @return true if the computed "order" property of aFrame1 is less than that
+ *         of aFrame2, or if the computed "order" values are equal and aFrame1's
+ *         corresponding DOM node is earlier than aFrame2's in the DOM tree.
+ *         Otherwise, returns false.
+ */
+bool
+IsOrderLEQWithDOMFallback(nsIFrame* aFrame1,
+                          nsIFrame* aFrame2)
+{
+  MOZ_ASSERT(aFrame1->IsFlexItem() && aFrame2->IsFlexItem(),
+             "this method only intended for comparing flex items");
+  MOZ_ASSERT(aFrame1->GetParent() == aFrame2->GetParent(),
+             "this method only intended for comparing siblings");
+  if (aFrame1 == aFrame2) {
+    // Anything is trivially LEQ itself, so we return "true" here... but it's
+    // probably bad if we end up actually needing this, so let's assert.
+    NS_ERROR("Why are we checking if a frame is LEQ itself?");
+    return true;
+  }
+
+  const bool isInLegacyBox = IsLegacyBox(aFrame1->GetParent());
+
+  int32_t order1 = GetOrderOrBoxOrdinalGroup(aFrame1, isInLegacyBox);
+  int32_t order2 = GetOrderOrBoxOrdinalGroup(aFrame2, isInLegacyBox);
+
+  if (order1 != order2) {
+    return order1 < order2;
+  }
+
+  // The "order" values are equal, so we need to fall back on DOM comparison.
+  // For that, we need to dig through any anonymous box wrapper frames to find
+  // the actual frame that corresponds to our child content.
+  aFrame1 = GetFirstNonAnonBoxDescendant(aFrame1);
+  aFrame2 = GetFirstNonAnonBoxDescendant(aFrame2);
+  MOZ_ASSERT(aFrame1 && aFrame2,
+             "why do we have an anonymous box without any "
+             "non-anonymous descendants?");
+
+
+  // Special case:
+  // If either frame is for generated content from ::before or ::after, then
+  // we can't use nsContentUtils::PositionIsBefore(), since that method won't
+  // recognize generated content as being an actual sibling of other nodes.
+  // We know where ::before and ::after nodes *effectively* insert in the DOM
+  // tree, though (at the beginning & end), so we can just special-case them.
+  nsIAtom* pseudo1 =
+    nsPlaceholderFrame::GetRealFrameFor(aFrame1)->StyleContext()->GetPseudo();
+  nsIAtom* pseudo2 =
+    nsPlaceholderFrame::GetRealFrameFor(aFrame2)->StyleContext()->GetPseudo();
+
+  if (pseudo1 == nsCSSPseudoElements::before ||
+      pseudo2 == nsCSSPseudoElements::after) {
+    // frame1 is ::before and/or frame2 is ::after => frame1 is LEQ frame2.
+    return true;
+  }
+  if (pseudo1 == nsCSSPseudoElements::after ||
+      pseudo2 == nsCSSPseudoElements::before) {
+    // frame1 is ::after and/or frame2 is ::before => frame1 is not LEQ frame2.
+    return false;
+  }
+
+  // Usual case: Compare DOM position.
+  nsIContent* content1 = aFrame1->GetContent();
+  nsIContent* content2 = aFrame2->GetContent();
+  MOZ_ASSERT(content1 != content2,
+             "Two different flex items are using the same nsIContent node for "
+             "comparison, so we may be sorting them in an arbitrary order");
+
+  return nsContentUtils::PositionIsBefore(content1, content2);
+}
+
+/**
+ * Sorting helper-function that compares two frames' "order" property-values.
+ * Returns true if aFrame1 is "less than or equal to" aFrame2 according to this
+ * comparison.
+ *
+ * Note: This can't be a static function, because we need to pass it as a
+ * template argument. (Only functions with external linkage can be passed as
+ * template arguments.)
+ *
+ * @return true if the computed "order" property of aFrame1 is less than or
+ *         equal to that of aFrame2.  Otherwise, returns false.
+ */
+bool
+IsOrderLEQ(nsIFrame* aFrame1,
+           nsIFrame* aFrame2)
+{
+  MOZ_ASSERT(aFrame1->IsFlexItem() && aFrame2->IsFlexItem(),
+             "this method only intended for comparing flex items");
+  MOZ_ASSERT(aFrame1->GetParent() == aFrame2->GetParent(),
+             "this method only intended for comparing siblings");
+
+  const bool isInLegacyBox = IsLegacyBox(aFrame1->GetParent());
+
+  int32_t order1 = GetOrderOrBoxOrdinalGroup(aFrame1, isInLegacyBox);
+  int32_t order2 = GetOrderOrBoxOrdinalGroup(aFrame2, isInLegacyBox);
+
+  return order1 <= order2;
+}
+
+uint8_t
+SimplifyAlignOrJustifyContentForOneItem(uint16_t aAlignmentVal,
+                                        bool aIsAlign)
+{
+  // Mask away any explicit fallback, to get the main (non-fallback) part of
+  // the specified value:
+  uint16_t specified = aAlignmentVal & NS_STYLE_ALIGN_ALL_BITS;
+
+  // XXX strip off <overflow-position> bits until we implement it (bug 1311892)
+  specified &= ~NS_STYLE_ALIGN_FLAG_BITS;
+
+  // FIRST: handle a special-case for "justify-content:stretch" (or equivalent),
+  // which requires that we ignore any author-provided explicit fallback value.
+  if (specified == NS_STYLE_ALIGN_NORMAL) {
+    // In a flex container, *-content: "'normal' behaves as 'stretch'".
+    // Do that conversion early, so it benefits from our 'stretch' special-case.
+    // https://drafts.csswg.org/css-align-3/#distribution-flex
+    specified = NS_STYLE_ALIGN_STRETCH;
+  }
+  if (!aIsAlign && specified == NS_STYLE_ALIGN_STRETCH) {
+    // In a flex container, in "justify-content Axis: [...] 'stretch' behaves
+    // as 'flex-start' (ignoring the specified fallback alignment, if any)."
+    // https://drafts.csswg.org/css-align-3/#distribution-flex
+    // So, we just directly return 'flex-start', & ignore explicit fallback..
+    return NS_STYLE_ALIGN_FLEX_START;
+  }
+
+  // Now check for an explicit fallback value (and if it's present, use it).
+  uint16_t explicitFallback = aAlignmentVal >> NS_STYLE_ALIGN_ALL_SHIFT;
+  if (explicitFallback) {
+    // XXX strip off <overflow-position> bits until we implement it
+    // (bug 1311892)
+    explicitFallback &= ~NS_STYLE_ALIGN_FLAG_BITS;
+    return explicitFallback;
+  }
+
+  // There's no explicit fallback. Use the implied fallback values for
+  // space-{between,around,evenly} (since those values only make sense with
+  // multiple alignment subjects), and otherwise just use the specified value:
+  switch (specified) {
+    case NS_STYLE_ALIGN_SPACE_BETWEEN:
+      return NS_STYLE_ALIGN_START;
+    case NS_STYLE_ALIGN_SPACE_AROUND:
+    case NS_STYLE_ALIGN_SPACE_EVENLY:
+      return NS_STYLE_ALIGN_CENTER;
+    default:
+      return specified;
+  }
+}
+
+uint16_t
+nsFlexContainerFrame::CSSAlignmentForAbsPosChild(
+  const ReflowInput& aChildRI,
+  LogicalAxis aLogicalAxis) const
+{
+  WritingMode wm = GetWritingMode();
+  const FlexboxAxisTracker
+    axisTracker(this, wm, AxisTrackerFlags::eAllowBottomToTopChildOrdering);
+
+  // If we're row-oriented and the caller is asking about our inline axis (or
+  // alternately, if we're column-oriented and the caller is asking about our
+  // block axis), then the caller is really asking about our *main* axis.
+  // Otherwise, the caller is asking about our cross axis.
+  const bool isMainAxis = (axisTracker.IsRowOriented() ==
+                           (aLogicalAxis == eLogicalAxisInline));
+  const nsStylePosition* containerStylePos = StylePosition();
+  const bool isAxisReversed = isMainAxis ? axisTracker.IsMainAxisReversed()
+                                         : axisTracker.IsCrossAxisReversed();
+
+  uint8_t alignment;
+  if (isMainAxis) {
+    alignment = SimplifyAlignOrJustifyContentForOneItem(
+                  containerStylePos->mJustifyContent,
+                  /*aIsAlign = */false);
+  } else {
+    const uint8_t alignContent = SimplifyAlignOrJustifyContentForOneItem(
+                                   containerStylePos->mAlignContent,
+                                   /*aIsAlign = */true);
+    if (NS_STYLE_FLEX_WRAP_NOWRAP != containerStylePos->mFlexWrap &&
+        alignContent != NS_STYLE_ALIGN_STRETCH) {
+      // Multi-line, align-content isn't stretch --> align-content determines
+      // this child's alignment in the cross axis.
+      alignment = alignContent;
+    } else {
+      // Single-line, or multi-line but the (one) line stretches to fill
+      // container. Respect align-self.
+      alignment = aChildRI.mStylePosition->UsedAlignSelf(StyleContext());
+      // XXX strip off <overflow-position> bits until we implement it
+      // (bug 1311892)
+      alignment &= ~NS_STYLE_ALIGN_FLAG_BITS;
+
+      if (alignment == NS_STYLE_ALIGN_NORMAL) {
+        // "the 'normal' keyword behaves as 'start' on replaced
+        // absolutely-positioned boxes, and behaves as 'stretch' on all other
+        // absolutely-positioned boxes."
+        // https://drafts.csswg.org/css-align/#align-abspos
+        alignment = aChildRI.mFrame->IsFrameOfType(nsIFrame::eReplaced) ?
+          NS_STYLE_ALIGN_START : NS_STYLE_ALIGN_STRETCH;
+      }
+    }
+  }
+
+  // Resolve flex-start, flex-end, auto, left, right, baseline, last baseline;
+  if (alignment == NS_STYLE_ALIGN_FLEX_START) {
+    alignment = isAxisReversed ? NS_STYLE_ALIGN_END : NS_STYLE_ALIGN_START;
+  } else if (alignment == NS_STYLE_ALIGN_FLEX_END) {
+    alignment = isAxisReversed ? NS_STYLE_ALIGN_START : NS_STYLE_ALIGN_END;
+  } else if (alignment == NS_STYLE_ALIGN_LEFT ||
+             alignment == NS_STYLE_ALIGN_RIGHT) {
+    if (aLogicalAxis == eLogicalAxisInline) {
+      const bool isLeft = (alignment == NS_STYLE_ALIGN_LEFT);
+      alignment = (isLeft == wm.IsBidiLTR()) ? NS_STYLE_ALIGN_START
+                                             : NS_STYLE_ALIGN_END;
+    } else {
+      alignment = NS_STYLE_ALIGN_START;
+    }
+  } else if (alignment == NS_STYLE_ALIGN_BASELINE) {
+    alignment = NS_STYLE_ALIGN_START;
+  } else if (alignment == NS_STYLE_ALIGN_LAST_BASELINE) {
+    alignment = NS_STYLE_ALIGN_END;
+  }
+
+  return alignment;
+}
+
+bool
+nsFlexContainerFrame::IsHorizontal()
+{
+  const FlexboxAxisTracker axisTracker(this, GetWritingMode());
+  return axisTracker.IsMainAxisHorizontal();
+}
+
+UniquePtr<FlexItem>
+nsFlexContainerFrame::GenerateFlexItemForChild(
+  nsPresContext* aPresContext,
+  nsIFrame*      aChildFrame,
+  const ReflowInput& aParentReflowInput,
+  const FlexboxAxisTracker& aAxisTracker)
+{
+  // Create temporary reflow state just for sizing -- to get hypothetical
+  // main-size and the computed values of min / max main-size property.
+  // (This reflow state will _not_ be used for reflow.)
+  ReflowInput
+    childRI(aPresContext, aParentReflowInput, aChildFrame,
+            aParentReflowInput.ComputedSize(aChildFrame->GetWritingMode()));
+
+  // FLEX GROW & SHRINK WEIGHTS
+  // --------------------------
+  float flexGrow, flexShrink;
+  if (IsLegacyBox(this)) {
+    flexGrow = flexShrink = aChildFrame->StyleXUL()->mBoxFlex;
+  } else {
+    const nsStylePosition* stylePos = aChildFrame->StylePosition();
+    flexGrow   = stylePos->mFlexGrow;
+    flexShrink = stylePos->mFlexShrink;
+  }
+
+  WritingMode childWM = childRI.GetWritingMode();
+
+  // MAIN SIZES (flex base size, min/max size)
+  // -----------------------------------------
+  nscoord flexBaseSize = GET_MAIN_COMPONENT_LOGICAL(aAxisTracker, childWM,
+                                                    childRI.ComputedISize(),
+                                                    childRI.ComputedBSize());
+  nscoord mainMinSize = GET_MAIN_COMPONENT_LOGICAL(aAxisTracker, childWM,
+                                                   childRI.ComputedMinISize(),
+                                                   childRI.ComputedMinBSize());
+  nscoord mainMaxSize = GET_MAIN_COMPONENT_LOGICAL(aAxisTracker, childWM,
+                                                   childRI.ComputedMaxISize(),
+                                                   childRI.ComputedMaxBSize());
+  // This is enforced by the ReflowInput where these values come from:
+  MOZ_ASSERT(mainMinSize <= mainMaxSize, "min size is larger than max size");
+
+  // CROSS SIZES (tentative cross size, min/max cross size)
+  // ------------------------------------------------------
+  // Grab the cross size from the reflow state. This might be the right value,
+  // or we might resolve it to something else in SizeItemInCrossAxis(); hence,
+  // it's tentative. See comment under "Cross Size Determination" for more.
+  nscoord tentativeCrossSize =
+    GET_CROSS_COMPONENT_LOGICAL(aAxisTracker, childWM,
+                                childRI.ComputedISize(),
+                                childRI.ComputedBSize());
+  nscoord crossMinSize =
+    GET_CROSS_COMPONENT_LOGICAL(aAxisTracker, childWM,
+                                childRI.ComputedMinISize(),
+                                childRI.ComputedMinBSize());
+  nscoord crossMaxSize =
+    GET_CROSS_COMPONENT_LOGICAL(aAxisTracker, childWM,
+                                childRI.ComputedMaxISize(),
+                                childRI.ComputedMaxBSize());
+
+  // SPECIAL-CASE FOR WIDGET-IMPOSED SIZES
+  // Check if we're a themed widget, in which case we might have a minimum
+  // main & cross size imposed by our widget (which we can't go below), or
+  // (more severe) our widget might have only a single valid size.
+  bool isFixedSizeWidget = false;
+  const nsStyleDisplay* disp = aChildFrame->StyleDisplay();
+  if (aChildFrame->IsThemed(disp)) {
+    LayoutDeviceIntSize widgetMinSize;
+    bool canOverride = true;
+    aPresContext->GetTheme()->
+      GetMinimumWidgetSize(aPresContext, aChildFrame,
+                           disp->mAppearance,
+                           &widgetMinSize, &canOverride);
+
+    nscoord widgetMainMinSize =
+      aPresContext->DevPixelsToAppUnits(
+        aAxisTracker.GetMainComponent(widgetMinSize));
+    nscoord widgetCrossMinSize =
+      aPresContext->DevPixelsToAppUnits(
+        aAxisTracker.GetCrossComponent(widgetMinSize));
+
+    // GMWS() returns border-box. We need content-box, so subtract
+    // borderPadding (but don't let that push our min sizes below 0).
+    nsMargin& bp = childRI.ComputedPhysicalBorderPadding();
+    widgetMainMinSize = std::max(widgetMainMinSize -
+                                 aAxisTracker.GetMarginSizeInMainAxis(bp), 0);
+    widgetCrossMinSize = std::max(widgetCrossMinSize -
+                                  aAxisTracker.GetMarginSizeInCrossAxis(bp), 0);
+
+    if (!canOverride) {
+      // Fixed-size widget: freeze our main-size at the widget's mandated size.
+      // (Set min and max main-sizes to that size, too, to keep us from
+      // clamping to any other size later on.)
+      flexBaseSize = mainMinSize = mainMaxSize = widgetMainMinSize;
+      tentativeCrossSize = crossMinSize = crossMaxSize = widgetCrossMinSize;
+      isFixedSizeWidget = true;
+    } else {
+      // Variable-size widget: ensure our min/max sizes are at least as large
+      // as the widget's mandated minimum size, so we don't flex below that.
+      mainMinSize = std::max(mainMinSize, widgetMainMinSize);
+      mainMaxSize = std::max(mainMaxSize, widgetMainMinSize);
+
+      if (tentativeCrossSize != NS_INTRINSICSIZE) {
+        tentativeCrossSize = std::max(tentativeCrossSize, widgetCrossMinSize);
+      }
+      crossMinSize = std::max(crossMinSize, widgetCrossMinSize);
+      crossMaxSize = std::max(crossMaxSize, widgetCrossMinSize);
+    }
+  }
+
+  // Construct the flex item!
+  auto item = MakeUnique<FlexItem>(childRI,
+                                   flexGrow, flexShrink, flexBaseSize,
+                                   mainMinSize, mainMaxSize,
+                                   tentativeCrossSize,
+                                   crossMinSize, crossMaxSize,
+                                   aAxisTracker);
+
+  // If we're inflexible, we can just freeze to our hypothetical main-size
+  // up-front. Similarly, if we're a fixed-size widget, we only have one
+  // valid size, so we freeze to keep ourselves from flexing.
+  if (isFixedSizeWidget || (flexGrow == 0.0f && flexShrink == 0.0f)) {
+    item->Freeze();
+  }
+
+  // Resolve "flex-basis:auto" and/or "min-[width|height]:auto" (which might
+  // require us to reflow the item to measure content height)
+  ResolveAutoFlexBasisAndMinSize(aPresContext, *item,
+                                 childRI, aAxisTracker);
+  return item;
+}
+
+// Static helper-functions for ResolveAutoFlexBasisAndMinSize():
+// -------------------------------------------------------------
+// Indicates whether the cross-size property is set to something definite.
+// The logic here should be similar to the logic for isAutoWidth/isAutoHeight
+// in nsFrame::ComputeSizeWithIntrinsicDimensions().
+static bool
+IsCrossSizeDefinite(const ReflowInput& aItemReflowInput,
+                    const FlexboxAxisTracker& aAxisTracker)
+{
+  const nsStylePosition* pos = aItemReflowInput.mStylePosition;
+  if (aAxisTracker.IsCrossAxisHorizontal()) {
+    return pos->mWidth.GetUnit() != eStyleUnit_Auto;
+  }
+  // else, vertical. (We need to use IsAutoHeight() to catch e.g. %-height
+  // applied to indefinite-height containing block, which counts as auto.)
+  nscoord cbHeight = aItemReflowInput.mCBReflowInput->ComputedHeight();
+  return !nsLayoutUtils::IsAutoHeight(pos->mHeight, cbHeight);
+}
+
+// If aFlexItem has a definite cross size, this function returns it, for usage
+// (in combination with an intrinsic ratio) for resolving the item's main size
+// or main min-size.
+//
+// The parameter "aMinSizeFallback" indicates whether we should fall back to
+// returning the cross min-size, when the cross size is indefinite. (This param
+// should be set IFF the caller intends to resolve the main min-size.) If this
+// param is true, then this function is guaranteed to return a definite value
+// (i.e. not NS_AUTOHEIGHT, excluding cases where huge sizes are involved).
+//
+// XXXdholbert the min-size behavior here is based on my understanding in
+//   http://lists.w3.org/Archives/Public/www-style/2014Jul/0053.html
+// If my understanding there ends up being wrong, we'll need to update this.
+static nscoord
+CrossSizeToUseWithRatio(const FlexItem& aFlexItem,
+                        const ReflowInput& aItemReflowInput,
+                        bool aMinSizeFallback,
+                        const FlexboxAxisTracker& aAxisTracker)
+{
+  if (aFlexItem.IsStretched()) {
+    // Definite cross-size, imposed via 'align-self:stretch' & flex container.
+    return aFlexItem.GetCrossSize();
+  }
+
+  if (IsCrossSizeDefinite(aItemReflowInput, aAxisTracker)) {
+    // Definite cross size.
+    return GET_CROSS_COMPONENT_LOGICAL(aAxisTracker, aFlexItem.GetWritingMode(),
+                                       aItemReflowInput.ComputedISize(),
+                                       aItemReflowInput.ComputedBSize());
+  }
+
+  if (aMinSizeFallback) {
+    // Indefinite cross-size, and we're resolving main min-size, so we'll fall
+    // back to ussing the cross min-size (which should be definite).
+    return GET_CROSS_COMPONENT_LOGICAL(aAxisTracker, aFlexItem.GetWritingMode(),
+                                       aItemReflowInput.ComputedMinISize(),
+                                       aItemReflowInput.ComputedMinBSize());
+  }
+
+  // Indefinite cross-size.
+  return NS_AUTOHEIGHT;
+}
+
+// Convenience function; returns a main-size, given a cross-size and an
+// intrinsic ratio. The intrinsic ratio must not have 0 in its cross-axis
+// component (or else we'll divide by 0).
+static nscoord
+MainSizeFromAspectRatio(nscoord aCrossSize,
+                        const nsSize& aIntrinsicRatio,
+                        const FlexboxAxisTracker& aAxisTracker)
+{
+  MOZ_ASSERT(aAxisTracker.GetCrossComponent(aIntrinsicRatio) != 0,
+             "Invalid ratio; will divide by 0! Caller should've checked...");
+
+  if (aAxisTracker.IsCrossAxisHorizontal()) {
+    // cross axis horiz --> aCrossSize is a width. Converting to height.
+    return NSCoordMulDiv(aCrossSize, aIntrinsicRatio.height, aIntrinsicRatio.width);
+  }
+  // cross axis vert --> aCrossSize is a height. Converting to width.
+  return NSCoordMulDiv(aCrossSize, aIntrinsicRatio.width, aIntrinsicRatio.height);
+}
+
+// Partially resolves "min-[width|height]:auto" and returns the resulting value.
+// By "partially", I mean we don't consider the min-content size (but we do
+// consider flex-basis, main max-size, and the intrinsic aspect ratio).
+// The caller is responsible for computing & considering the min-content size
+// in combination with the partially-resolved value that this function returns.
+//
+// Spec reference: http://dev.w3.org/csswg/css-flexbox/#min-size-auto
+static nscoord
+PartiallyResolveAutoMinSize(const FlexItem& aFlexItem,
+                            const ReflowInput& aItemReflowInput,
+                            const FlexboxAxisTracker& aAxisTracker)
+{
+  MOZ_ASSERT(aFlexItem.NeedsMinSizeAutoResolution(),
+             "only call for FlexItems that need min-size auto resolution");
+
+  nscoord minMainSize = nscoord_MAX; // Intentionally huge; we'll shrink it
+                                     // from here, w/ std::min().
+
+  // We need the smallest of:
+  // * the used flex-basis, if the computed flex-basis was 'auto':
+  // XXXdholbert ('auto' might be renamed to 'main-size'; see bug 1032922)
+  if (eStyleUnit_Auto ==
+      aItemReflowInput.mStylePosition->mFlexBasis.GetUnit() &&
+      aFlexItem.GetFlexBaseSize() != NS_AUTOHEIGHT) {
+    // NOTE: We skip this if the flex base size depends on content & isn't yet
+    // resolved. This is OK, because the caller is responsible for computing
+    // the min-content height and min()'ing it with the value we return, which
+    // is equivalent to what would happen if we min()'d that at this point.
+    minMainSize = std::min(minMainSize, aFlexItem.GetFlexBaseSize());
+  }
+
+  // * the computed max-width (max-height), if that value is definite:
+  nscoord maxSize =
+    GET_MAIN_COMPONENT_LOGICAL(aAxisTracker, aFlexItem.GetWritingMode(),
+                               aItemReflowInput.ComputedMaxISize(),
+                               aItemReflowInput.ComputedMaxBSize());
+  if (maxSize != NS_UNCONSTRAINEDSIZE) {
+    minMainSize = std::min(minMainSize, maxSize);
+  }
+
+  // * if the item has no intrinsic aspect ratio, its min-content size:
+  //  --- SKIPPING THIS IN THIS FUNCTION --- caller's responsibility.
+
+  // * if the item has an intrinsic aspect ratio, the width (height) calculated
+  //   from the aspect ratio and any definite size constraints in the opposite
+  //   dimension.
+  if (aAxisTracker.GetCrossComponent(aFlexItem.IntrinsicRatio()) != 0) {
+    // We have a usable aspect ratio. (not going to divide by 0)
+    const bool useMinSizeIfCrossSizeIsIndefinite = true;
+    nscoord crossSizeToUseWithRatio =
+      CrossSizeToUseWithRatio(aFlexItem, aItemReflowInput,
+                              useMinSizeIfCrossSizeIsIndefinite,
+                              aAxisTracker);
+    nscoord minMainSizeFromRatio =
+      MainSizeFromAspectRatio(crossSizeToUseWithRatio,
+                              aFlexItem.IntrinsicRatio(), aAxisTracker);
+    minMainSize = std::min(minMainSize, minMainSizeFromRatio);
+  }
+
+  return minMainSize;
+}
+
+// Resolves flex-basis:auto, using the given intrinsic ratio and the flex
+// item's cross size.  On success, updates the flex item with its resolved
+// flex-basis and returns true. On failure (e.g. if the ratio is invalid or
+// the cross-size is indefinite), returns false.
+static bool
+ResolveAutoFlexBasisFromRatio(FlexItem& aFlexItem,
+                              const ReflowInput& aItemReflowInput,
+                              const FlexboxAxisTracker& aAxisTracker)
+{
+  MOZ_ASSERT(NS_AUTOHEIGHT == aFlexItem.GetFlexBaseSize(),
+             "Should only be called to resolve an 'auto' flex-basis");
+  // If the flex item has ...
+  //  - an intrinsic aspect ratio,
+  //  - a [used] flex-basis of 'main-size' [auto?] [We have this, if we're here.]
+  //  - a definite cross size
+  // then the flex base size is calculated from its inner cross size and the
+  // flex item’s intrinsic aspect ratio.
+  if (aAxisTracker.GetCrossComponent(aFlexItem.IntrinsicRatio()) != 0) {
+    // We have a usable aspect ratio. (not going to divide by 0)
+    const bool useMinSizeIfCrossSizeIsIndefinite = false;
+    nscoord crossSizeToUseWithRatio =
+      CrossSizeToUseWithRatio(aFlexItem, aItemReflowInput,
+                              useMinSizeIfCrossSizeIsIndefinite,
+                              aAxisTracker);
+    if (crossSizeToUseWithRatio != NS_AUTOHEIGHT) {
+      // We have a definite cross-size
+      nscoord mainSizeFromRatio =
+        MainSizeFromAspectRatio(crossSizeToUseWithRatio,
+                                aFlexItem.IntrinsicRatio(), aAxisTracker);
+      aFlexItem.SetFlexBaseSizeAndMainSize(mainSizeFromRatio);
+      return true;
+    }
+  }
+  return false;
+}
+
+// Note: If & when we handle "min-height: min-content" for flex items,
+// we may want to resolve that in this function, too.
+void
+nsFlexContainerFrame::
+  ResolveAutoFlexBasisAndMinSize(nsPresContext* aPresContext,
+                                 FlexItem& aFlexItem,
+                                 const ReflowInput& aItemReflowInput,
+                                 const FlexboxAxisTracker& aAxisTracker)
+{
+  // (Note: We should never have a used flex-basis of "auto" if our main axis
+  // is horizontal; width values should always be resolvable without reflow.)
+  const bool isMainSizeAuto = (!aAxisTracker.IsMainAxisHorizontal() &&
+                               NS_AUTOHEIGHT == aFlexItem.GetFlexBaseSize());
+
+  const bool isMainMinSizeAuto = aFlexItem.NeedsMinSizeAutoResolution();
+
+  if (!isMainSizeAuto && !isMainMinSizeAuto) {
+    // Nothing to do; this function is only needed for flex items
+    // with a used flex-basis of "auto" or a min-main-size of "auto".
+    return;
+  }
+
+  // We may be about to do computations based on our item's cross-size
+  // (e.g. using it as a contstraint when measuring our content in the
+  // main axis, or using it with the intrinsic ratio to obtain a main size).
+  // BEFORE WE DO THAT, we need let the item "pre-stretch" its cross size (if
+  // it's got 'align-self:stretch'), for a certain case where the spec says
+  // the stretched cross size is considered "definite". That case is if we
+  // have a single-line (nowrap) flex container which itself has a definite
+  // cross-size.  Otherwise, we'll wait to do stretching, since (in other
+  // cases) we don't know how much the item should stretch yet.
+  const ReflowInput* flexContainerRI = aItemReflowInput.mParentReflowInput;
+  MOZ_ASSERT(flexContainerRI,
+             "flex item's reflow state should have ptr to container's state");
+  if (NS_STYLE_FLEX_WRAP_NOWRAP == flexContainerRI->mStylePosition->mFlexWrap) {
+    // XXXdholbert Maybe this should share logic with ComputeCrossSize()...
+    // Alternately, maybe tentative container cross size should be passed down.
+    nscoord containerCrossSize =
+      GET_CROSS_COMPONENT_LOGICAL(aAxisTracker, aAxisTracker.GetWritingMode(),
+                                  flexContainerRI->ComputedISize(),
+                                  flexContainerRI->ComputedBSize());
+    // Is container's cross size "definite"?
+    // (Container's cross size is definite if cross-axis is horizontal, or if
+    // cross-axis is vertical and the cross-size is not NS_AUTOHEIGHT.)
+    if (aAxisTracker.IsCrossAxisHorizontal() ||
+        containerCrossSize != NS_AUTOHEIGHT) {
+      aFlexItem.ResolveStretchedCrossSize(containerCrossSize, aAxisTracker);
+    }
+  }
+
+  nscoord resolvedMinSize; // (only set/used if isMainMinSizeAuto==true)
+  bool minSizeNeedsToMeasureContent = false; // assume the best
+  if (isMainMinSizeAuto) {
+    // Resolve the min-size, except for considering the min-content size.
+    // (We'll consider that later, if we need to.)
+    resolvedMinSize = PartiallyResolveAutoMinSize(aFlexItem, aItemReflowInput,
+                                                  aAxisTracker);
+    if (resolvedMinSize > 0 &&
+        aAxisTracker.GetCrossComponent(aFlexItem.IntrinsicRatio()) == 0) {
+      // We don't have a usable aspect ratio, so we need to consider our
+      // min-content size as another candidate min-size, which we'll have to
+      // min() with the current resolvedMinSize.
+      // (If resolvedMinSize were already at 0, we could skip this measurement
+      // because it can't go any lower. But it's not 0, so we need it.)
+      minSizeNeedsToMeasureContent = true;
+    }
+  }
+
+  bool flexBasisNeedsToMeasureContent = false; // assume the best
+  if (isMainSizeAuto) {
+    if (!ResolveAutoFlexBasisFromRatio(aFlexItem, aItemReflowInput,
+                                       aAxisTracker)) {
+      flexBasisNeedsToMeasureContent = true;
+    }
+  }
+
+  // Measure content, if needed (w/ intrinsic-width method or a reflow)
+  if (minSizeNeedsToMeasureContent || flexBasisNeedsToMeasureContent) {
+    if (aAxisTracker.IsMainAxisHorizontal()) {
+      if (minSizeNeedsToMeasureContent) {
+        nscoord frameMinISize =
+          aFlexItem.Frame()->GetMinISize(aItemReflowInput.mRenderingContext);
+        resolvedMinSize = std::min(resolvedMinSize, frameMinISize);
+      }
+      NS_ASSERTION(!flexBasisNeedsToMeasureContent,
+                   "flex-basis:auto should have been resolved in the "
+                   "reflow state, for horizontal flexbox. It shouldn't need "
+                   "special handling here");
+    } else {
+      // If this item is flexible (vertically), or if we're measuring the
+      // 'auto' min-height and our main-size is something else, then we assume
+      // that the computed-height we're reflowing with now could be different
+      // from the one we'll use for this flex item's "actual" reflow later on.
+      // In that case, we need to be sure the flex item treats this as a
+      // vertical resize, even though none of its ancestors are necessarily
+      // being vertically resized.
+      // (Note: We don't have to do this for width, because InitResizeFlags
+      // will always turn on mHResize on when it sees that the computed width
+      // is different from current width, and that's all we need.)
+      bool forceVerticalResizeForMeasuringReflow =
+        !aFlexItem.IsFrozen() ||         // Is the item flexible?
+        !flexBasisNeedsToMeasureContent; // Are we *only* measuring it for
+                                         // 'min-height:auto'?
+
+      nscoord contentHeight =
+        MeasureFlexItemContentHeight(aPresContext, aFlexItem,
+                                     forceVerticalResizeForMeasuringReflow,
+                                     *flexContainerRI);
+      if (minSizeNeedsToMeasureContent) {
+        resolvedMinSize = std::min(resolvedMinSize, contentHeight);
+      }
+      if (flexBasisNeedsToMeasureContent) {
+        aFlexItem.SetFlexBaseSizeAndMainSize(contentHeight);
+      }
+    }
+  }
+
+  if (isMainMinSizeAuto) {
+    aFlexItem.UpdateMainMinSize(resolvedMinSize);
+  }
+}
+
+nscoord
+nsFlexContainerFrame::
+  MeasureFlexItemContentHeight(nsPresContext* aPresContext,
+                               FlexItem& aFlexItem,
+                               bool aForceVerticalResizeForMeasuringReflow,
+                               const ReflowInput& aParentReflowInput)
+{
+  // Set up a reflow state for measuring the flex item's auto-height:
+  WritingMode wm = aFlexItem.Frame()->GetWritingMode();
+  LogicalSize availSize = aParentReflowInput.ComputedSize(wm);
+  availSize.BSize(wm) = NS_UNCONSTRAINEDSIZE;
+  ReflowInput
+    childRIForMeasuringHeight(aPresContext, aParentReflowInput,
+                              aFlexItem.Frame(), availSize,
+                              nullptr, ReflowInput::CALLER_WILL_INIT);
+  childRIForMeasuringHeight.mFlags.mIsFlexContainerMeasuringHeight = true;
+  childRIForMeasuringHeight.Init(aPresContext);
+
+  if (aFlexItem.IsStretched()) {
+    childRIForMeasuringHeight.SetComputedWidth(aFlexItem.GetCrossSize());
+    childRIForMeasuringHeight.SetHResize(true);
+  }
+
+  if (aForceVerticalResizeForMeasuringReflow) {
+    childRIForMeasuringHeight.SetVResize(true);
+  }
+
+  ReflowOutput childDesiredSize(childRIForMeasuringHeight);
+  nsReflowStatus childReflowStatus;
+  const uint32_t flags = NS_FRAME_NO_MOVE_FRAME;
+  ReflowChild(aFlexItem.Frame(), aPresContext,
+              childDesiredSize, childRIForMeasuringHeight,
+              0, 0, flags, childReflowStatus);
+
+  MOZ_ASSERT(NS_FRAME_IS_COMPLETE(childReflowStatus),
+             "We gave flex item unconstrained available height, so it "
+             "should be complete");
+
+  FinishReflowChild(aFlexItem.Frame(), aPresContext,
+                    childDesiredSize, &childRIForMeasuringHeight,
+                    0, 0, flags);
+
+  aFlexItem.SetHadMeasuringReflow();
+  aFlexItem.SetAscent(childDesiredSize.BlockStartAscent());
+
+  // Subtract border/padding in vertical axis, to get _just_
+  // the effective computed value of the "height" property.
+  nscoord childDesiredHeight = childDesiredSize.Height() -
+    childRIForMeasuringHeight.ComputedPhysicalBorderPadding().TopBottom();
+
+  return std::max(0, childDesiredHeight);
+}
+
+FlexItem::FlexItem(ReflowInput& aFlexItemReflowInput,
+                   float aFlexGrow, float aFlexShrink, nscoord aFlexBaseSize,
+                   nscoord aMainMinSize,  nscoord aMainMaxSize,
+                   nscoord aTentativeCrossSize,
+                   nscoord aCrossMinSize, nscoord aCrossMaxSize,
+                   const FlexboxAxisTracker& aAxisTracker)
+  : mFrame(aFlexItemReflowInput.mFrame),
+    mFlexGrow(aFlexGrow),
+    mFlexShrink(aFlexShrink),
+    mIntrinsicRatio(mFrame->GetIntrinsicRatio()),
+    mBorderPadding(aFlexItemReflowInput.ComputedPhysicalBorderPadding()),
+    mMargin(aFlexItemReflowInput.ComputedPhysicalMargin()),
+    mMainMinSize(aMainMinSize),
+    mMainMaxSize(aMainMaxSize),
+    mCrossMinSize(aCrossMinSize),
+    mCrossMaxSize(aCrossMaxSize),
+    mMainPosn(0),
+    mCrossSize(aTentativeCrossSize),
+    mCrossPosn(0),
+    mAscent(0),
+    mShareOfWeightSoFar(0.0f),
+    mIsFrozen(false),
+    mHadMinViolation(false),
+    mHadMaxViolation(false),
+    mHadMeasuringReflow(false),
+    mIsStretched(false),
+    mIsStrut(false),
+    // mNeedsMinSizeAutoResolution is initialized in CheckForMinSizeAuto()
+    mWM(aFlexItemReflowInput.GetWritingMode())
+    // mAlignSelf, see below
+{
+  MOZ_ASSERT(mFrame, "expecting a non-null child frame");
+  MOZ_ASSERT(mFrame->GetType() != nsGkAtoms::placeholderFrame,
+             "placeholder frames should not be treated as flex items");
+  MOZ_ASSERT(!(mFrame->GetStateBits() & NS_FRAME_OUT_OF_FLOW),
+             "out-of-flow frames should not be treated as flex items");
+
+  const ReflowInput* containerRS = aFlexItemReflowInput.mParentReflowInput;
+  if (IsLegacyBox(containerRS->mFrame)) {
+    // For -webkit-box/-webkit-inline-box, we need to:
+    // (1) Use "-webkit-box-align" instead of "align-items" to determine the
+    //     container's cross-axis alignment behavior.
+    // (2) Suppress the ability for flex items to override that with their own
+    //     cross-axis alignment. (The legacy box model doesn't support this.)
+    // So, each FlexItem simply copies the container's converted "align-items"
+    // value and disregards their own "align-self" property.
+    const nsStyleXUL* containerStyleXUL = containerRS->mFrame->StyleXUL();
+    mAlignSelf = ConvertLegacyStyleToAlignItems(containerStyleXUL);
+  } else {
+    mAlignSelf = aFlexItemReflowInput.mStylePosition->UsedAlignSelf(
+                   containerRS->mFrame->StyleContext());
+    if (MOZ_LIKELY(mAlignSelf == NS_STYLE_ALIGN_NORMAL)) {
+      mAlignSelf = NS_STYLE_ALIGN_STRETCH;
+    }
+
+    // XXX strip off the <overflow-position> bit until we implement that
+    mAlignSelf &= ~NS_STYLE_ALIGN_FLAG_BITS;
+  }
+
+  SetFlexBaseSizeAndMainSize(aFlexBaseSize);
+  CheckForMinSizeAuto(aFlexItemReflowInput, aAxisTracker);
+
+  // Assert that any "auto" margin components are set to 0.
+  // (We'll resolve them later; until then, we want to treat them as 0-sized.)
+#ifdef DEBUG
+  {
+    const nsStyleSides& styleMargin =
+      aFlexItemReflowInput.mStyleMargin->mMargin;
+    NS_FOR_CSS_SIDES(side) {
+      if (styleMargin.GetUnit(side) == eStyleUnit_Auto) {
+        MOZ_ASSERT(GetMarginComponentForSide(side) == 0,
+                   "Someone else tried to resolve our auto margin");
+      }
+    }
+  }
+#endif // DEBUG
+
+  // Map align-self 'baseline' value to 'start' when baseline alignment
+  // is not possible because the FlexItem's writing mode is orthogonal to
+  // the main axis of the container. If that's the case, we just directly
+  // convert our align-self value here, so that we don't have to handle this
+  // with special cases elsewhere.
+  // We are treating this case as one where it is appropriate to use the
+  // fallback values defined at https://www.w3.org/TR/css-align-3/#baseline
+  if (aAxisTracker.IsRowOriented() ==
+      aAxisTracker.GetWritingMode().IsOrthogonalTo(mWM)) {
+    if (mAlignSelf == NS_STYLE_ALIGN_BASELINE) {
+      mAlignSelf = NS_STYLE_ALIGN_FLEX_START;
+    } else if (mAlignSelf == NS_STYLE_ALIGN_LAST_BASELINE) {
+      mAlignSelf = NS_STYLE_ALIGN_FLEX_END;
+    }
+  }
+}
+
+// Simplified constructor for creating a special "strut" FlexItem, for a child
+// with visibility:collapse. The strut has 0 main-size, and it only exists to
+// impose a minimum cross size on whichever FlexLine it ends up in.
+FlexItem::FlexItem(nsIFrame* aChildFrame, nscoord aCrossSize,
+                   WritingMode aContainerWM)
+  : mFrame(aChildFrame),
+    mFlexGrow(0.0f),
+    mFlexShrink(0.0f),
+    mIntrinsicRatio(),
+    // mBorderPadding uses default constructor,
+    // mMargin uses default constructor,
+    mFlexBaseSize(0),
+    mMainMinSize(0),
+    mMainMaxSize(0),
+    mCrossMinSize(0),
+    mCrossMaxSize(0),
+    mMainSize(0),
+    mMainPosn(0),
+    mCrossSize(aCrossSize),
+    mCrossPosn(0),
+    mAscent(0),
+    mShareOfWeightSoFar(0.0f),
+    mIsFrozen(true),
+    mHadMinViolation(false),
+    mHadMaxViolation(false),
+    mHadMeasuringReflow(false),
+    mIsStretched(false),
+    mIsStrut(true), // (this is the constructor for making struts, after all)
+    mNeedsMinSizeAutoResolution(false),
+    mWM(aContainerWM),
+    mAlignSelf(NS_STYLE_ALIGN_FLEX_START)
+{
+  MOZ_ASSERT(mFrame, "expecting a non-null child frame");
+  MOZ_ASSERT(NS_STYLE_VISIBILITY_COLLAPSE ==
+             mFrame->StyleVisibility()->mVisible,
+             "Should only make struts for children with 'visibility:collapse'");
+  MOZ_ASSERT(mFrame->GetType() != nsGkAtoms::placeholderFrame,
+             "placeholder frames should not be treated as flex items");
+  MOZ_ASSERT(!(mFrame->GetStateBits() & NS_FRAME_OUT_OF_FLOW),
+             "out-of-flow frames should not be treated as flex items");
+}
+
+void
+FlexItem::CheckForMinSizeAuto(const ReflowInput& aFlexItemReflowInput,
+                              const FlexboxAxisTracker& aAxisTracker)
+{
+  const nsStylePosition* pos = aFlexItemReflowInput.mStylePosition;
+  const nsStyleDisplay* disp = aFlexItemReflowInput.mStyleDisplay;
+
+  // We'll need special behavior for "min-[width|height]:auto" (whichever is in
+  // the main axis) iff:
+  // (a) its computed value is "auto"
+  // (b) the "overflow" sub-property in the same axis (the main axis) has a
+  //     computed value of "visible"
+  const nsStyleCoord& minSize = GET_MAIN_COMPONENT(aAxisTracker,
+                                                   pos->mMinWidth,
+                                                   pos->mMinHeight);
+
+  const uint8_t overflowVal = GET_MAIN_COMPONENT(aAxisTracker,
+                                                 disp->mOverflowX,
+                                                 disp->mOverflowY);
+
+  mNeedsMinSizeAutoResolution = (minSize.GetUnit() == eStyleUnit_Auto &&
+                                 overflowVal == NS_STYLE_OVERFLOW_VISIBLE);
+}
+
+nscoord
+FlexItem::GetBaselineOffsetFromOuterCrossEdge(
+  AxisEdgeType aEdge,
+  const FlexboxAxisTracker& aAxisTracker,
+  bool aUseFirstLineBaseline) const
+{
+  // NOTE: Currently, 'mAscent' (taken from reflow) is an inherently vertical
+  // measurement -- it's the distance from the border-top edge of this FlexItem
+  // to its baseline. So, we can really only do baseline alignment when the
+  // cross axis is vertical. (The FlexItem constructor enforces this when
+  // resolving the item's "mAlignSelf" value).
+  MOZ_ASSERT(!aAxisTracker.IsCrossAxisHorizontal(),
+             "Only expecting to be doing baseline computations when the "
+             "cross axis is vertical");
+
+  AxisOrientationType crossAxis = aAxisTracker.GetCrossAxis();
+  mozilla::Side sideToMeasureFrom = kAxisOrientationToSidesMap[crossAxis][aEdge];
+
+  nscoord marginTopToBaseline = ResolvedAscent(aUseFirstLineBaseline) +
+                                mMargin.top;
+
+  if (sideToMeasureFrom == eSideTop) {
+    // Measuring from top (normal case): the distance from the margin-box top
+    // edge to the baseline is just ascent + margin-top.
+    return marginTopToBaseline;
+  }
+
+  MOZ_ASSERT(sideToMeasureFrom == eSideBottom,
+             "We already checked that we're dealing with a vertical axis, and "
+             "we're not using the top side, so that only leaves the bottom...");
+
+  // Measuring from bottom: The distance from the margin-box bottom edge to the
+  // baseline is just the margin-box cross size (i.e. outer cross size), minus
+  // the already-computed distance from margin-top to baseline.
+  return GetOuterCrossSize(crossAxis) - marginTopToBaseline;
+}
+
+uint32_t
+FlexItem::GetNumAutoMarginsInAxis(AxisOrientationType aAxis) const
+{
+  uint32_t numAutoMargins = 0;
+  const nsStyleSides& styleMargin = mFrame->StyleMargin()->mMargin;
+  for (uint32_t i = 0; i < eNumAxisEdges; i++) {
+    mozilla::Side side = kAxisOrientationToSidesMap[aAxis][i];
+    if (styleMargin.GetUnit(side) == eStyleUnit_Auto) {
+      numAutoMargins++;
+    }
+  }
+
+  // Mostly for clarity:
+  MOZ_ASSERT(numAutoMargins <= 2,
+             "We're just looking at one item along one dimension, so we "
+             "should only have examined 2 margins");
+
+  return numAutoMargins;
+}
+
+bool
+FlexItem::CanMainSizeInfluenceCrossSize(
+  const FlexboxAxisTracker& aAxisTracker) const
+{
+  if (mIsStretched) {
+    // We've already had our cross-size stretched for "align-self:stretch").
+    // The container is imposing its cross size on us.
+    return false;
+  }
+
+  if (mIsStrut) {
+    // Struts (for visibility:collapse items) have a predetermined size;
+    // no need to measure anything.
+    return false;
+  }
+
+  if (HasIntrinsicRatio()) {
+    // For flex items that have an intrinsic ratio (and maintain it, i.e. are
+    // not stretched, which we already checked above): changes to main-size
+    // *do* influence the cross size.
+    return true;
+  }
+
+  if (aAxisTracker.IsCrossAxisHorizontal()) {
+    // If the cross axis is horizontal, then changes to the item's main size
+    // (height) can't influence its cross size (width), if the item is a block
+    // with a horizontal writing-mode.
+    // XXXdholbert This doesn't account for vertical writing-modes, items with
+    // aspect ratios, items that are multicol elements, & items that are
+    // multi-line vertical flex containers. In all of those cases, a change to
+    // the height could influence the width.
+    return false;
+  }
+
+  // Default assumption, if we haven't proven otherwise: the resolved main size
+  // *can* change the cross size.
+  return true;
+}
+
+// Keeps track of our position along a particular axis (where a '0' position
+// corresponds to the 'start' edge of that axis).
+// This class shouldn't be instantiated directly -- rather, it should only be
+// instantiated via its subclasses defined below.
+class MOZ_STACK_CLASS PositionTracker {
+public:
+  // Accessor for the current value of the position that we're tracking.
+  inline nscoord GetPosition() const { return mPosition; }
+  inline AxisOrientationType GetAxis() const { return mAxis; }
+
+  // Advances our position across the start edge of the given margin, in the
+  // axis we're tracking.
+  void EnterMargin(const nsMargin& aMargin)
+  {
+    mozilla::Side side = kAxisOrientationToSidesMap[mAxis][eAxisEdge_Start];
+    mPosition += aMargin.Side(side);
+  }
+
+  // Advances our position across the end edge of the given margin, in the axis
+  // we're tracking.
+  void ExitMargin(const nsMargin& aMargin)
+  {
+    mozilla::Side side = kAxisOrientationToSidesMap[mAxis][eAxisEdge_End];
+    mPosition += aMargin.Side(side);
+  }
+
+  // Advances our current position from the start side of a child frame's
+  // border-box to the frame's upper or left edge (depending on our axis).
+  // (Note that this is a no-op if our axis grows in the same direction as
+  // the corresponding logical axis.)
+  void EnterChildFrame(nscoord aChildFrameSize)
+  {
+    if (mIsAxisReversed) {
+      mPosition += aChildFrameSize;
+    }
+  }
+
+  // Advances our current position from a frame's upper or left border-box edge
+  // (whichever is in the axis we're tracking) to the 'end' side of the frame
+  // in the axis that we're tracking. (Note that this is a no-op if our axis
+  // is reversed with respect to the corresponding logical axis.)
+  void ExitChildFrame(nscoord aChildFrameSize)
+  {
+    if (!mIsAxisReversed) {
+      mPosition += aChildFrameSize;
+    }
+  }
+
+protected:
+  // Protected constructor, to be sure we're only instantiated via a subclass.
+  PositionTracker(AxisOrientationType aAxis, bool aIsAxisReversed)
+    : mPosition(0),
+      mAxis(aAxis),
+      mIsAxisReversed(aIsAxisReversed)
+  {}
+
+  // Delete copy-constructor & reassignment operator, to prevent accidental
+  // (unnecessary) copying.
+  PositionTracker(const PositionTracker&) = delete;
+  PositionTracker& operator=(const PositionTracker&) = delete;
+
+  // Member data:
+  nscoord mPosition;               // The position we're tracking
+  // XXXdholbert [BEGIN DEPRECATED]
+  const AxisOrientationType mAxis; // The axis along which we're moving.
+  // XXXdholbert [END DEPRECATED]
+  const bool mIsAxisReversed; // Is the axis along which we're moving reversed
+                              // (e.g. LTR vs RTL) with respect to the
+                              // corresponding axis on the flex container's WM?
+};
+
+// Tracks our position in the main axis, when we're laying out flex items.
+// The "0" position represents the main-start edge of the flex container's
+// content-box.
+class MOZ_STACK_CLASS MainAxisPositionTracker : public PositionTracker {
+public:
+  MainAxisPositionTracker(const FlexboxAxisTracker& aAxisTracker,
+                          const FlexLine* aLine,
+                          uint8_t aJustifyContent,
+                          nscoord aContentBoxMainSize);
+
+  ~MainAxisPositionTracker() {
+    MOZ_ASSERT(mNumPackingSpacesRemaining == 0,
+               "miscounted the number of packing spaces");
+    MOZ_ASSERT(mNumAutoMarginsInMainAxis == 0,
+               "miscounted the number of auto margins");
+  }
+
+  // Advances past the packing space (if any) between two flex items
+  void TraversePackingSpace();
+
+  // If aItem has any 'auto' margins in the main axis, this method updates the
+  // corresponding values in its margin.
+  void ResolveAutoMarginsInMainAxis(FlexItem& aItem);
+
+private:
+  nscoord  mPackingSpaceRemaining;
+  uint32_t mNumAutoMarginsInMainAxis;
+  uint32_t mNumPackingSpacesRemaining;
+  // XXX this should be uint16_t when we add explicit fallback handling
+  uint8_t  mJustifyContent;
+};
+
+// Utility class for managing our position along the cross axis along
+// the whole flex container (at a higher level than a single line).
+// The "0" position represents the cross-start edge of the flex container's
+// content-box.
+class MOZ_STACK_CLASS CrossAxisPositionTracker : public PositionTracker {
+public:
+  CrossAxisPositionTracker(FlexLine* aFirstLine,
+                           const ReflowInput& aReflowInput,
+                           nscoord aContentBoxCrossSize,
+                           bool aIsCrossSizeDefinite,
+                           const FlexboxAxisTracker& aAxisTracker);
+
+  // Advances past the packing space (if any) between two flex lines
+  void TraversePackingSpace();
+
+  // Advances past the given FlexLine
+  void TraverseLine(FlexLine& aLine) { mPosition += aLine.GetLineCrossSize(); }
+
+private:
+  // Redeclare the frame-related methods from PositionTracker as private with
+  // = delete, to be sure (at compile time) that no client code can invoke
+  // them. (Unlike the other PositionTracker derived classes, this class here
+  // deals with FlexLines, not with individual FlexItems or frames.)
+  void EnterMargin(const nsMargin& aMargin) = delete;
+  void ExitMargin(const nsMargin& aMargin) = delete;
+  void EnterChildFrame(nscoord aChildFrameSize) = delete;
+  void ExitChildFrame(nscoord aChildFrameSize) = delete;
+
+  nscoord  mPackingSpaceRemaining;
+  uint32_t mNumPackingSpacesRemaining;
+  // XXX this should be uint16_t when we add explicit fallback handling
+  uint8_t  mAlignContent;
+};
+
+// Utility class for managing our position along the cross axis, *within* a
+// single flex line.
+class MOZ_STACK_CLASS SingleLineCrossAxisPositionTracker : public PositionTracker {
+public:
+  explicit SingleLineCrossAxisPositionTracker(const FlexboxAxisTracker& aAxisTracker);
+
+  void ResolveAutoMarginsInCrossAxis(const FlexLine& aLine,
+                                     FlexItem& aItem);
+
+  void EnterAlignPackingSpace(const FlexLine& aLine,
+                              const FlexItem& aItem,
+                              const FlexboxAxisTracker& aAxisTracker);
+
+  // Resets our position to the cross-start edge of this line.
+  inline void ResetPosition() { mPosition = 0; }
+};
+
+//----------------------------------------------------------------------
+
+// Frame class boilerplate
+// =======================
+
+NS_QUERYFRAME_HEAD(nsFlexContainerFrame)
+  NS_QUERYFRAME_ENTRY(nsFlexContainerFrame)
+NS_QUERYFRAME_TAIL_INHERITING(nsContainerFrame)
+
+NS_IMPL_FRAMEARENA_HELPERS(nsFlexContainerFrame)
+
+nsContainerFrame*
+NS_NewFlexContainerFrame(nsIPresShell* aPresShell,
+                         nsStyleContext* aContext)
+{
+  return new (aPresShell) nsFlexContainerFrame(aContext);
+}
+
+//----------------------------------------------------------------------
+
+// nsFlexContainerFrame Method Implementations
+// ===========================================
+
+/* virtual */
+nsFlexContainerFrame::~nsFlexContainerFrame()
+{
+}
+
+/* virtual */
+void
+nsFlexContainerFrame::Init(nsIContent*       aContent,
+                           nsContainerFrame* aParent,
+                           nsIFrame*         aPrevInFlow)
+{
+  nsContainerFrame::Init(aContent, aParent, aPrevInFlow);
+
+  const nsStyleDisplay* styleDisp = StyleContext()->StyleDisplay();
+
+  // Figure out if we should set a frame state bit to indicate that this frame
+  // represents a legacy -webkit-{inline-}box container.
+  // First, the trivial case: just check "display" directly.
+  bool isLegacyBox = IsDisplayValueLegacyBox(styleDisp);
+
+  // If this frame is for a scrollable element, then it will actually have
+  // "display:block", and its *parent* will have the real flex-flavored display
+  // value. So in that case, check the parent to find out if we're legacy.
+  if (!isLegacyBox && styleDisp->mDisplay == mozilla::StyleDisplay::Block) {
+    nsStyleContext* parentStyleContext = mStyleContext->GetParent();
+    NS_ASSERTION(parentStyleContext &&
+                 (mStyleContext->GetPseudo() == nsCSSAnonBoxes::buttonContent ||
+                  mStyleContext->GetPseudo() == nsCSSAnonBoxes::scrolledContent),
+                 "The only way a nsFlexContainerFrame can have 'display:block' "
+                 "should be if it's the inner part of a scrollable or button "
+                 "element");
+    isLegacyBox = IsDisplayValueLegacyBox(parentStyleContext->StyleDisplay());
+  }
+
+  if (isLegacyBox) {
+    AddStateBits(NS_STATE_FLEX_IS_LEGACY_WEBKIT_BOX);
+  }
+}
+
+template<bool IsLessThanOrEqual(nsIFrame*, nsIFrame*)>
+/* static */ bool
+nsFlexContainerFrame::SortChildrenIfNeeded()
+{
+  if (nsIFrame::IsFrameListSorted<IsLessThanOrEqual>(mFrames)) {
+    return false;
+  }
+
+  nsIFrame::SortFrameList<IsLessThanOrEqual>(mFrames);
+  return true;
+}
+
+/* virtual */
+nsIAtom*
+nsFlexContainerFrame::GetType() const
+{
+  return nsGkAtoms::flexContainerFrame;
+}
+
+#ifdef DEBUG_FRAME_DUMP
+nsresult
+nsFlexContainerFrame::GetFrameName(nsAString& aResult) const
+{
+  return MakeFrameName(NS_LITERAL_STRING("FlexContainer"), aResult);
+}
+#endif
+
+nscoord
+nsFlexContainerFrame::GetLogicalBaseline(mozilla::WritingMode aWM) const
+{
+  NS_ASSERTION(mBaselineFromLastReflow != NS_INTRINSIC_WIDTH_UNKNOWN,
+               "baseline has not been set");
+
+  if (HasAnyStateBits(NS_STATE_FLEX_SYNTHESIZE_BASELINE)) {
+    // Return a baseline synthesized from our margin-box.
+    return nsContainerFrame::GetLogicalBaseline(aWM);
+  }
+  return mBaselineFromLastReflow;
+}
+
+// Helper for BuildDisplayList, to implement this special-case for flex items
+// from the spec:
+//    Flex items paint exactly the same as block-level elements in the
+//    normal flow, except that 'z-index' values other than 'auto' create
+//    a stacking context even if 'position' is 'static'.
+// http://www.w3.org/TR/2012/CR-css3-flexbox-20120918/#painting
+uint32_t
+GetDisplayFlagsForFlexItem(nsIFrame* aFrame)
+{
+  MOZ_ASSERT(aFrame->IsFlexItem(), "Should only be called on flex items");
+
+  const nsStylePosition* pos = aFrame->StylePosition();
+  if (pos->mZIndex.GetUnit() == eStyleUnit_Integer) {
+    return nsIFrame::DISPLAY_CHILD_FORCE_STACKING_CONTEXT;
+  }
+  return nsIFrame::DISPLAY_CHILD_FORCE_PSEUDO_STACKING_CONTEXT;
+}
+
+void
+nsFlexContainerFrame::BuildDisplayList(nsDisplayListBuilder*   aBuilder,
+                                       const nsRect&           aDirtyRect,
+                                       const nsDisplayListSet& aLists)
+{
+  // XXXdholbert hacky temporary band-aid for bug 1059138: Trivially pass this
+  // assertion (skip it, basically) if the first child is part of a shadow DOM.
+  // (IsOrderLEQWithDOMFallback doesn't know how to compare tree-position of a
+  // shadow-DOM element vs. a non-shadow-DOM element.)
+  NS_ASSERTION(
+    (!mFrames.IsEmpty() &&
+     mFrames.FirstChild()->GetContent()->GetContainingShadow()) ||
+    nsIFrame::IsFrameListSorted<IsOrderLEQWithDOMFallback>(mFrames),
+    "Child frames aren't sorted correctly");
+
+  DisplayBorderBackgroundOutline(aBuilder, aLists);
+
+  // Our children are all block-level, so their borders/backgrounds all go on
+  // the BlockBorderBackgrounds list.
+  nsDisplayListSet childLists(aLists, aLists.BlockBorderBackgrounds());
+  for (nsIFrame* childFrame : mFrames) {
+    BuildDisplayListForChild(aBuilder, childFrame, aDirtyRect, childLists,
+                             GetDisplayFlagsForFlexItem(childFrame));
+  }
+}
+
+void
+FlexLine::FreezeItemsEarly(bool aIsUsingFlexGrow)
+{
+  // After we've established the type of flexing we're doing (growing vs.
+  // shrinking), and before we try to flex any items, we freeze items that
+  // obviously *can't* flex.
+  //
+  // Quoting the spec:
+  //  # Freeze, setting its target main size to its hypothetical main size...
+  //  #  - any item that has a flex factor of zero
+  //  #  - if using the flex grow factor: any item that has a flex base size
+  //  #    greater than its hypothetical main size
+  //  #  - if using the flex shrink factor: any item that has a flex base size
+  //  #    smaller than its hypothetical main size
+  //  http://dev.w3.org/csswg/css-flexbox/#resolve-flexible-lengths-flex-factors
+  //
+  // (NOTE: At this point, item->GetMainSize() *is* the item's hypothetical
+  // main size, since SetFlexBaseSizeAndMainSize() sets it up that way, and the
+  // item hasn't had a chance to flex away from that yet.)
+
+  // Since this loop only operates on unfrozen flex items, we can break as
+  // soon as we have seen all of them.
+  uint32_t numUnfrozenItemsToBeSeen = mNumItems - mNumFrozenItems;
+  for (FlexItem* item = mItems.getFirst();
+       numUnfrozenItemsToBeSeen > 0; item = item->getNext()) {
+    MOZ_ASSERT(item, "numUnfrozenItemsToBeSeen says items remain to be seen");
+
+    if (!item->IsFrozen()) {
+      numUnfrozenItemsToBeSeen--;
+      bool shouldFreeze = (0.0f == item->GetFlexFactor(aIsUsingFlexGrow));
+      if (!shouldFreeze) {
+        if (aIsUsingFlexGrow) {
+          if (item->GetFlexBaseSize() > item->GetMainSize()) {
+            shouldFreeze = true;
+          }
+        } else { // using flex-shrink
+          if (item->GetFlexBaseSize() < item->GetMainSize()) {
+            shouldFreeze = true;
+          }
+        }
+      }
+      if (shouldFreeze) {
+        // Freeze item! (at its hypothetical main size)
+        item->Freeze();
+        mNumFrozenItems++;
+      }
+    }
+  }
+}
+
+// Based on the sign of aTotalViolation, this function freezes a subset of our
+// flexible sizes, and restores the remaining ones to their initial pref sizes.
+void
+FlexLine::FreezeOrRestoreEachFlexibleSize(const nscoord aTotalViolation,
+                                          bool aIsFinalIteration)
+{
+  enum FreezeType {
+    eFreezeEverything,
+    eFreezeMinViolations,
+    eFreezeMaxViolations
+  };
+
+  FreezeType freezeType;
+  if (aTotalViolation == 0) {
+    freezeType = eFreezeEverything;
+  } else if (aTotalViolation > 0) {
+    freezeType = eFreezeMinViolations;
+  } else { // aTotalViolation < 0
+    freezeType = eFreezeMaxViolations;
+  }
+
+  // Since this loop only operates on unfrozen flex items, we can break as
+  // soon as we have seen all of them.
+  uint32_t numUnfrozenItemsToBeSeen = mNumItems - mNumFrozenItems;
+  for (FlexItem* item = mItems.getFirst();
+       numUnfrozenItemsToBeSeen > 0; item = item->getNext()) {
+    MOZ_ASSERT(item, "numUnfrozenItemsToBeSeen says items remain to be seen");
+    if (!item->IsFrozen()) {
+      numUnfrozenItemsToBeSeen--;
+
+      MOZ_ASSERT(!item->HadMinViolation() || !item->HadMaxViolation(),
+                 "Can have either min or max violation, but not both");
+
+      if (eFreezeEverything == freezeType ||
+          (eFreezeMinViolations == freezeType && item->HadMinViolation()) ||
+          (eFreezeMaxViolations == freezeType && item->HadMaxViolation())) {
+
+        MOZ_ASSERT(item->GetMainSize() >= item->GetMainMinSize(),
+                   "Freezing item at a size below its minimum");
+        MOZ_ASSERT(item->GetMainSize() <= item->GetMainMaxSize(),
+                   "Freezing item at a size above its maximum");
+
+        item->Freeze();
+        mNumFrozenItems++;
+      } else if (MOZ_UNLIKELY(aIsFinalIteration)) {
+        // XXXdholbert If & when bug 765861 is fixed, we should upgrade this
+        // assertion to be fatal except in documents with enormous lengths.
+        NS_ERROR("Final iteration still has unfrozen items, this shouldn't"
+                 " happen unless there was nscoord under/overflow.");
+        item->Freeze();
+        mNumFrozenItems++;
+      } // else, we'll reset this item's main size to its flex base size on the
+        // next iteration of this algorithm.
+
+      // Clear this item's violation(s), now that we've dealt with them
+      item->ClearViolationFlags();
+    }
+  }
+}
+
+void
+FlexLine::ResolveFlexibleLengths(nscoord aFlexContainerMainSize)
+{
+  MOZ_LOG(gFlexContainerLog, LogLevel::Debug, ("ResolveFlexibleLengths\n"));
+
+  // Determine whether we're going to be growing or shrinking items.
+  const bool isUsingFlexGrow =
+    (mTotalOuterHypotheticalMainSize < aFlexContainerMainSize);
+
+  // Do an "early freeze" for flex items that obviously can't flex in the
+  // direction we've chosen:
+  FreezeItemsEarly(isUsingFlexGrow);
+
+  if (mNumFrozenItems == mNumItems) {
+    // All our items are frozen, so we have no flexible lengths to resolve.
+    return;
+  }
+  MOZ_ASSERT(!IsEmpty(), "empty lines should take the early-return above");
+
+  // Subtract space occupied by our items' margins/borders/padding, so we can
+  // just be dealing with the space available for our flex items' content
+  // boxes.
+  nscoord spaceReservedForMarginBorderPadding =
+    mTotalOuterHypotheticalMainSize - mTotalInnerHypotheticalMainSize;
+
+  nscoord spaceAvailableForFlexItemsContentBoxes =
+    aFlexContainerMainSize - spaceReservedForMarginBorderPadding;
+
+  nscoord origAvailableFreeSpace;
+  bool isOrigAvailFreeSpaceInitialized = false;
+
+  // NOTE: I claim that this chunk of the algorithm (the looping part) needs to
+  // run the loop at MOST mNumItems times.  This claim should hold up
+  // because we'll freeze at least one item on each loop iteration, and once
+  // we've run out of items to freeze, there's nothing left to do.  However,
+  // in most cases, we'll break out of this loop long before we hit that many
+  // iterations.
+  for (uint32_t iterationCounter = 0;
+       iterationCounter < mNumItems; iterationCounter++) {
+    // Set every not-yet-frozen item's used main size to its
+    // flex base size, and subtract all the used main sizes from our
+    // total amount of space to determine the 'available free space'
+    // (positive or negative) to be distributed among our flexible items.
+    nscoord availableFreeSpace = spaceAvailableForFlexItemsContentBoxes;
+    for (FlexItem* item = mItems.getFirst(); item; item = item->getNext()) {
+      if (!item->IsFrozen()) {
+        item->SetMainSize(item->GetFlexBaseSize());
+      }
+      availableFreeSpace -= item->GetMainSize();
+    }
+
+    MOZ_LOG(gFlexContainerLog, LogLevel::Debug,
+           (" available free space = %d\n", availableFreeSpace));
+
+
+    // The sign of our free space should agree with the type of flexing
+    // (grow/shrink) that we're doing (except if we've had integer overflow;
+    // then, all bets are off). Any disagreement should've made us use the
+    // other type of flexing, or should've been resolved in FreezeItemsEarly.
+    // XXXdholbert If & when bug 765861 is fixed, we should upgrade this
+    // assertion to be fatal except in documents with enormous lengths.
+    NS_ASSERTION((isUsingFlexGrow && availableFreeSpace >= 0) ||
+                 (!isUsingFlexGrow && availableFreeSpace <= 0),
+                 "availableFreeSpace's sign should match isUsingFlexGrow");
+
+    // If we have any free space available, give each flexible item a portion
+    // of availableFreeSpace.
+    if (availableFreeSpace != 0) {
+      // The first time we do this, we initialize origAvailableFreeSpace.
+      if (!isOrigAvailFreeSpaceInitialized) {
+        origAvailableFreeSpace = availableFreeSpace;
+        isOrigAvailFreeSpaceInitialized = true;
+      }
+
+      // STRATEGY: On each item, we compute & store its "share" of the total
+      // weight that we've seen so far:
+      //   curWeight / weightSum
+      //
+      // Then, when we go to actually distribute the space (in the next loop),
+      // we can simply walk backwards through the elements and give each item
+      // its "share" multiplied by the remaining available space.
+      //
+      // SPECIAL CASE: If the sum of the weights is larger than the
+      // maximum representable float (overflowing to infinity), then we can't
+      // sensibly divide out proportional shares anymore. In that case, we
+      // simply treat the flex item(s) with the largest weights as if
+      // their weights were infinite (dwarfing all the others), and we
+      // distribute all of the available space among them.
+      float weightSum = 0.0f;
+      float flexFactorSum = 0.0f;
+      float largestWeight = 0.0f;
+      uint32_t numItemsWithLargestWeight = 0;
+
+      // Since this loop only operates on unfrozen flex items, we can break as
+      // soon as we have seen all of them.
+      uint32_t numUnfrozenItemsToBeSeen = mNumItems - mNumFrozenItems;
+      for (FlexItem* item = mItems.getFirst();
+           numUnfrozenItemsToBeSeen > 0; item = item->getNext()) {
+        MOZ_ASSERT(item,
+                   "numUnfrozenItemsToBeSeen says items remain to be seen");
+        if (!item->IsFrozen()) {
+          numUnfrozenItemsToBeSeen--;
+
+          float curWeight = item->GetWeight(isUsingFlexGrow);
+          float curFlexFactor = item->GetFlexFactor(isUsingFlexGrow);
+          MOZ_ASSERT(curWeight >= 0.0f, "weights are non-negative");
+          MOZ_ASSERT(curFlexFactor >= 0.0f, "flex factors are non-negative");
+
+          weightSum += curWeight;
+          flexFactorSum += curFlexFactor;
+
+          if (IsFinite(weightSum)) {
+            if (curWeight == 0.0f) {
+              item->SetShareOfWeightSoFar(0.0f);
+            } else {
+              item->SetShareOfWeightSoFar(curWeight / weightSum);
+            }
+          } // else, the sum of weights overflows to infinity, in which
+            // case we don't bother with "SetShareOfWeightSoFar" since
+            // we know we won't use it. (instead, we'll just give every
+            // item with the largest weight an equal share of space.)
+
+          // Update our largest-weight tracking vars
+          if (curWeight > largestWeight) {
+            largestWeight = curWeight;
+            numItemsWithLargestWeight = 1;
+          } else if (curWeight == largestWeight) {
+            numItemsWithLargestWeight++;
+          }
+        }
+      }
+
+      if (weightSum != 0.0f) {
+        MOZ_ASSERT(flexFactorSum != 0.0f,
+                   "flex factor sum can't be 0, if a weighted sum "
+                   "of its components (weightSum) is nonzero");
+        if (flexFactorSum < 1.0f) {
+          // Our unfrozen flex items don't want all of the original free space!
+          // (Their flex factors add up to something less than 1.)
+          // Hence, make sure we don't distribute any more than the portion of
+          // our original free space that these items actually want.
+          nscoord totalDesiredPortionOfOrigFreeSpace =
+            NSToCoordRound(origAvailableFreeSpace * flexFactorSum);
+
+          // Clamp availableFreeSpace to be no larger than that ^^.
+          // (using min or max, depending on sign).
+          // This should not change the sign of availableFreeSpace (except
+          // possibly by setting it to 0), as enforced by this assertion:
+          MOZ_ASSERT(totalDesiredPortionOfOrigFreeSpace == 0 ||
+                     ((totalDesiredPortionOfOrigFreeSpace > 0) ==
+                      (availableFreeSpace > 0)),
+                     "When we reduce available free space for flex factors < 1,"
+                     "we shouldn't change the sign of the free space...");
+
+          if (availableFreeSpace > 0) {
+            availableFreeSpace = std::min(availableFreeSpace,
+                                          totalDesiredPortionOfOrigFreeSpace);
+          } else {
+            availableFreeSpace = std::max(availableFreeSpace,
+                                          totalDesiredPortionOfOrigFreeSpace);
+          }
+        }
+
+        MOZ_LOG(gFlexContainerLog, LogLevel::Debug,
+               (" Distributing available space:"));
+        // Since this loop only operates on unfrozen flex items, we can break as
+        // soon as we have seen all of them.
+        numUnfrozenItemsToBeSeen = mNumItems - mNumFrozenItems;
+
+        // NOTE: It's important that we traverse our items in *reverse* order
+        // here, for correct width distribution according to the items'
+        // "ShareOfWeightSoFar" progressively-calculated values.
+        for (FlexItem* item = mItems.getLast();
+             numUnfrozenItemsToBeSeen > 0; item = item->getPrevious()) {
+          MOZ_ASSERT(item,
+                     "numUnfrozenItemsToBeSeen says items remain to be seen");
+          if (!item->IsFrozen()) {
+            numUnfrozenItemsToBeSeen--;
+
+            // To avoid rounding issues, we compute the change in size for this
+            // item, and then subtract it from the remaining available space.
+            nscoord sizeDelta = 0;
+            if (IsFinite(weightSum)) {
+              float myShareOfRemainingSpace =
+                item->GetShareOfWeightSoFar();
+
+              MOZ_ASSERT(myShareOfRemainingSpace >= 0.0f &&
+                         myShareOfRemainingSpace <= 1.0f,
+                         "my share should be nonnegative fractional amount");
+
+              if (myShareOfRemainingSpace == 1.0f) {
+                // (We special-case 1.0f to avoid float error from converting
+                // availableFreeSpace from integer*1.0f --> float --> integer)
+                sizeDelta = availableFreeSpace;
+              } else if (myShareOfRemainingSpace > 0.0f) {
+                sizeDelta = NSToCoordRound(availableFreeSpace *
+                                           myShareOfRemainingSpace);
+              }
+            } else if (item->GetWeight(isUsingFlexGrow) == largestWeight) {
+              // Total flexibility is infinite, so we're just distributing
+              // the available space equally among the items that are tied for
+              // having the largest weight (and this is one of those items).
+              sizeDelta =
+                NSToCoordRound(availableFreeSpace /
+                               float(numItemsWithLargestWeight));
+              numItemsWithLargestWeight--;
+            }
+
+            availableFreeSpace -= sizeDelta;
+
+            item->SetMainSize(item->GetMainSize() + sizeDelta);
+            MOZ_LOG(gFlexContainerLog, LogLevel::Debug,
+                   ("  child %p receives %d, for a total of %d\n",
+                    item, sizeDelta, item->GetMainSize()));
+          }
+        }
+      }
+    }
+
+    // Fix min/max violations:
+    nscoord totalViolation = 0; // keeps track of adjustments for min/max
+    MOZ_LOG(gFlexContainerLog, LogLevel::Debug,
+           (" Checking for violations:"));
+
+    // Since this loop only operates on unfrozen flex items, we can break as
+    // soon as we have seen all of them.
+    uint32_t numUnfrozenItemsToBeSeen = mNumItems - mNumFrozenItems;
+    for (FlexItem* item = mItems.getFirst();
+         numUnfrozenItemsToBeSeen > 0; item = item->getNext()) {
+      MOZ_ASSERT(item, "numUnfrozenItemsToBeSeen says items remain to be seen");
+      if (!item->IsFrozen()) {
+        numUnfrozenItemsToBeSeen--;
+
+        if (item->GetMainSize() < item->GetMainMinSize()) {
+          // min violation
+          totalViolation += item->GetMainMinSize() - item->GetMainSize();
+          item->SetMainSize(item->GetMainMinSize());
+          item->SetHadMinViolation();
+        } else if (item->GetMainSize() > item->GetMainMaxSize()) {
+          // max violation
+          totalViolation += item->GetMainMaxSize() - item->GetMainSize();
+          item->SetMainSize(item->GetMainMaxSize());
+          item->SetHadMaxViolation();
+        }
+      }
+    }
+
+    FreezeOrRestoreEachFlexibleSize(totalViolation,
+                                    iterationCounter + 1 == mNumItems);
+
+    MOZ_LOG(gFlexContainerLog, LogLevel::Debug,
+           (" Total violation: %d\n", totalViolation));
+
+    if (mNumFrozenItems == mNumItems) {
+      break;
+    }
+
+    MOZ_ASSERT(totalViolation != 0,
+               "Zero violation should've made us freeze all items & break");
+  }
+
+#ifdef DEBUG
+  // Post-condition: all items should've been frozen.
+  // Make sure the counts match:
+  MOZ_ASSERT(mNumFrozenItems == mNumItems, "All items should be frozen");
+
+  // For good measure, check each item directly, in case our counts are busted:
+  for (const FlexItem* item = mItems.getFirst(); item; item = item->getNext()) {
+    MOZ_ASSERT(item->IsFrozen(), "All items should be frozen");
+  }
+#endif // DEBUG
+}
+
+MainAxisPositionTracker::
+  MainAxisPositionTracker(const FlexboxAxisTracker& aAxisTracker,
+                          const FlexLine* aLine,
+                          uint8_t aJustifyContent,
+                          nscoord aContentBoxMainSize)
+  : PositionTracker(aAxisTracker.GetMainAxis(),
+                    aAxisTracker.IsMainAxisReversed()),
+    mPackingSpaceRemaining(aContentBoxMainSize), // we chip away at this below
+    mNumAutoMarginsInMainAxis(0),
+    mNumPackingSpacesRemaining(0),
+    mJustifyContent(aJustifyContent)
+{
+  // 'normal' behaves as 'stretch', and 'stretch' behaves as 'flex-start',
+  // in the main axis
+  // https://drafts.csswg.org/css-align-3/#propdef-justify-content
+  if (mJustifyContent == NS_STYLE_JUSTIFY_NORMAL ||
+      mJustifyContent == NS_STYLE_JUSTIFY_STRETCH) {
+    mJustifyContent = NS_STYLE_JUSTIFY_FLEX_START;
+  }
+
+  // XXX strip off the <overflow-position> bit until we implement that
+  mJustifyContent &= ~NS_STYLE_JUSTIFY_FLAG_BITS;
+
+  // mPackingSpaceRemaining is initialized to the container's main size.  Now
+  // we'll subtract out the main sizes of our flex items, so that it ends up
+  // with the *actual* amount of packing space.
+  for (const FlexItem* item = aLine->GetFirstItem(); item;
+       item = item->getNext()) {
+    mPackingSpaceRemaining -= item->GetOuterMainSize(mAxis);
+    mNumAutoMarginsInMainAxis += item->GetNumAutoMarginsInAxis(mAxis);
+  }
+
+  if (mPackingSpaceRemaining <= 0) {
+    // No available packing space to use for resolving auto margins.
+    mNumAutoMarginsInMainAxis = 0;
+  }
+
+  // If packing space is negative, 'space-between' falls back to 'flex-start',
+  // and 'space-around' & 'space-evenly' fall back to 'center'. In those cases,
+  // it's simplest to just pretend we have a different 'justify-content' value
+  // and share code.
+  if (mPackingSpaceRemaining < 0) {
+    if (mJustifyContent == NS_STYLE_JUSTIFY_SPACE_BETWEEN) {
+      mJustifyContent = NS_STYLE_JUSTIFY_FLEX_START;
+    } else if (mJustifyContent == NS_STYLE_JUSTIFY_SPACE_AROUND ||
+               mJustifyContent == NS_STYLE_JUSTIFY_SPACE_EVENLY) {
+      mJustifyContent = NS_STYLE_JUSTIFY_CENTER;
+    }
+  }
+
+  // Map 'left'/'right' to 'start'/'end'
+  if (mJustifyContent == NS_STYLE_JUSTIFY_LEFT ||
+      mJustifyContent == NS_STYLE_JUSTIFY_RIGHT) {
+    if (aAxisTracker.IsColumnOriented()) {
+      // Container's alignment axis is not parallel to the inline axis,
+      // so we map both 'left' and 'right' to 'start'.
+      mJustifyContent = NS_STYLE_JUSTIFY_START;
+    } else {
+      // Row-oriented, so we map 'left' and 'right' to 'start' or 'end',
+      // depending on left-to-right writing mode.
+      const bool isLTR = aAxisTracker.GetWritingMode().IsBidiLTR();
+      const bool isJustifyLeft = (mJustifyContent == NS_STYLE_JUSTIFY_LEFT);
+      mJustifyContent = (isJustifyLeft == isLTR) ? NS_STYLE_JUSTIFY_START
+                                                 : NS_STYLE_JUSTIFY_END;
+    }
+  }
+
+  // Map 'start'/'end' to 'flex-start'/'flex-end'.
+  if (mJustifyContent == NS_STYLE_JUSTIFY_START) {
+    mJustifyContent = NS_STYLE_JUSTIFY_FLEX_START;
+  } else if (mJustifyContent == NS_STYLE_JUSTIFY_END) {
+    mJustifyContent = NS_STYLE_JUSTIFY_FLEX_END;
+  }
+
+  // If our main axis is (internally) reversed, swap the justify-content
+  // "flex-start" and "flex-end" behaviors:
+  if (aAxisTracker.AreAxesInternallyReversed()) {
+    if (mJustifyContent == NS_STYLE_JUSTIFY_FLEX_START) {
+      mJustifyContent = NS_STYLE_JUSTIFY_FLEX_END;
+    } else if (mJustifyContent == NS_STYLE_JUSTIFY_FLEX_END) {
+      mJustifyContent = NS_STYLE_JUSTIFY_FLEX_START;
+    }
+  }
+
+  // Figure out how much space we'll set aside for auto margins or
+  // packing spaces, and advance past any leading packing-space.
+  if (mNumAutoMarginsInMainAxis == 0 &&
+      mPackingSpaceRemaining != 0 &&
+      !aLine->IsEmpty()) {
+    switch (mJustifyContent) {
+      case NS_STYLE_JUSTIFY_BASELINE:
+      case NS_STYLE_JUSTIFY_LAST_BASELINE:
+        NS_WARNING("NYI: justify-content:left/right/baseline/last baseline");
+        MOZ_FALLTHROUGH;
+      case NS_STYLE_JUSTIFY_FLEX_START:
+        // All packing space should go at the end --> nothing to do here.
+        break;
+      case NS_STYLE_JUSTIFY_FLEX_END:
+        // All packing space goes at the beginning
+        mPosition += mPackingSpaceRemaining;
+        break;
+      case NS_STYLE_JUSTIFY_CENTER:
+        // Half the packing space goes at the beginning
+        mPosition += mPackingSpaceRemaining / 2;
+        break;
+      case NS_STYLE_JUSTIFY_SPACE_BETWEEN:
+      case NS_STYLE_JUSTIFY_SPACE_AROUND:
+      case NS_STYLE_JUSTIFY_SPACE_EVENLY:
+        nsFlexContainerFrame::CalculatePackingSpace(aLine->NumItems(),
+                                                    mJustifyContent,
+                                                    &mPosition,
+                                                    &mNumPackingSpacesRemaining,
+                                                    &mPackingSpaceRemaining);
+        break;
+      default:
+        MOZ_ASSERT_UNREACHABLE("Unexpected justify-content value");
+    }
+  }
+
+  MOZ_ASSERT(mNumPackingSpacesRemaining == 0 ||
+             mNumAutoMarginsInMainAxis == 0,
+             "extra space should either go to packing space or to "
+             "auto margins, but not to both");
+}
+
+void
+MainAxisPositionTracker::ResolveAutoMarginsInMainAxis(FlexItem& aItem)
+{
+  if (mNumAutoMarginsInMainAxis) {
+    const nsStyleSides& styleMargin = aItem.Frame()->StyleMargin()->mMargin;
+    for (uint32_t i = 0; i < eNumAxisEdges; i++) {
+      mozilla::Side side = kAxisOrientationToSidesMap[mAxis][i];
+      if (styleMargin.GetUnit(side) == eStyleUnit_Auto) {
+        // NOTE: This integer math will skew the distribution of remainder
+        // app-units towards the end, which is fine.
+        nscoord curAutoMarginSize =
+          mPackingSpaceRemaining / mNumAutoMarginsInMainAxis;
+
+        MOZ_ASSERT(aItem.GetMarginComponentForSide(side) == 0,
+                   "Expecting auto margins to have value '0' before we "
+                   "resolve them");
+        aItem.SetMarginComponentForSide(side, curAutoMarginSize);
+
+        mNumAutoMarginsInMainAxis--;
+        mPackingSpaceRemaining -= curAutoMarginSize;
+      }
+    }
+  }
+}
+
+void
+MainAxisPositionTracker::TraversePackingSpace()
+{
+  if (mNumPackingSpacesRemaining) {
+    MOZ_ASSERT(mJustifyContent == NS_STYLE_JUSTIFY_SPACE_BETWEEN ||
+               mJustifyContent == NS_STYLE_JUSTIFY_SPACE_AROUND ||
+               mJustifyContent == NS_STYLE_JUSTIFY_SPACE_EVENLY,
+               "mNumPackingSpacesRemaining only applies for "
+               "space-between/space-around/space-evenly");
+
+    MOZ_ASSERT(mPackingSpaceRemaining >= 0,
+               "ran out of packing space earlier than we expected");
+
+    // NOTE: This integer math will skew the distribution of remainder
+    // app-units towards the end, which is fine.
+    nscoord curPackingSpace =
+      mPackingSpaceRemaining / mNumPackingSpacesRemaining;
+
+    mPosition += curPackingSpace;
+    mNumPackingSpacesRemaining--;
+    mPackingSpaceRemaining -= curPackingSpace;
+  }
+}
+
+CrossAxisPositionTracker::
+  CrossAxisPositionTracker(FlexLine* aFirstLine,
+                           const ReflowInput& aReflowInput,
+                           nscoord aContentBoxCrossSize,
+                           bool aIsCrossSizeDefinite,
+                           const FlexboxAxisTracker& aAxisTracker)
+  : PositionTracker(aAxisTracker.GetCrossAxis(),
+                    aAxisTracker.IsCrossAxisReversed()),
+    mPackingSpaceRemaining(0),
+    mNumPackingSpacesRemaining(0),
+    mAlignContent(aReflowInput.mStylePosition->mAlignContent)
+{
+  MOZ_ASSERT(aFirstLine, "null first line pointer");
+
+  // 'normal' behaves as 'stretch'
+  if (mAlignContent == NS_STYLE_ALIGN_NORMAL) {
+    mAlignContent = NS_STYLE_ALIGN_STRETCH;
+  }
+
+  // XXX strip of the <overflow-position> bit until we implement that
+  mAlignContent &= ~NS_STYLE_ALIGN_FLAG_BITS;
+
+  const bool isSingleLine =
+    NS_STYLE_FLEX_WRAP_NOWRAP == aReflowInput.mStylePosition->mFlexWrap;
+  if (isSingleLine) {
+    MOZ_ASSERT(!aFirstLine->getNext(),
+               "If we're styled as single-line, we should only have 1 line");
+    // "If the flex container is single-line and has a definite cross size, the
+    // cross size of the flex line is the flex container's inner cross size."
+    //
+    // SOURCE: https://drafts.csswg.org/css-flexbox/#algo-cross-line
+    // NOTE: This means (by definition) that there's no packing space, which
+    // means we don't need to be concerned with "align-conent" at all and we
+    // can return early. This is handy, because this is the usual case (for
+    // single-line flexbox).
+    if (aIsCrossSizeDefinite) {
+      aFirstLine->SetLineCrossSize(aContentBoxCrossSize);
+      return;
+    }
+
+    // "If the flex container is single-line, then clamp the line's
+    // cross-size to be within the container's computed min and max cross-size
+    // properties."
+    aFirstLine->SetLineCrossSize(NS_CSS_MINMAX(aFirstLine->GetLineCrossSize(),
+                                               aReflowInput.ComputedMinBSize(),
+                                               aReflowInput.ComputedMaxBSize()));
+  }
+
+  // NOTE: The rest of this function should essentially match
+  // MainAxisPositionTracker's constructor, though with FlexLines instead of
+  // FlexItems, and with the additional value "stretch" (and of course with
+  // cross sizes instead of main sizes.)
+
+  // Figure out how much packing space we have (container's cross size minus
+  // all the lines' cross sizes).  Also, share this loop to count how many
+  // lines we have. (We need that count in some cases below.)
+  mPackingSpaceRemaining = aContentBoxCrossSize;
+  uint32_t numLines = 0;
+  for (FlexLine* line = aFirstLine; line; line = line->getNext()) {
+    mPackingSpaceRemaining -= line->GetLineCrossSize();
+    numLines++;
+  }
+
+  // If packing space is negative, 'space-between' and 'stretch' behave like
+  // 'flex-start', and 'space-around' and 'space-evenly' behave like 'center'.
+  // In those cases, it's simplest to just pretend we have a different
+  // 'align-content' value and share code.
+  if (mPackingSpaceRemaining < 0) {
+    if (mAlignContent == NS_STYLE_ALIGN_SPACE_BETWEEN ||
+        mAlignContent == NS_STYLE_ALIGN_STRETCH) {
+      mAlignContent = NS_STYLE_ALIGN_FLEX_START;
+    } else if (mAlignContent == NS_STYLE_ALIGN_SPACE_AROUND ||
+               mAlignContent == NS_STYLE_ALIGN_SPACE_EVENLY) {
+      mAlignContent = NS_STYLE_ALIGN_CENTER;
+    }
+  }
+
+  // Map 'left'/'right' to 'start'/'end'
+  if (mAlignContent == NS_STYLE_ALIGN_LEFT ||
+      mAlignContent == NS_STYLE_ALIGN_RIGHT) {
+    if (aAxisTracker.IsRowOriented()) {
+      // Container's alignment axis is not parallel to the inline axis,
+      // so we map both 'left' and 'right' to 'start'.
+      mAlignContent = NS_STYLE_ALIGN_START;
+    } else {
+      // Column-oriented, so we map 'left' and 'right' to 'start' or 'end',
+      // depending on left-to-right writing mode.
+      const bool isLTR = aAxisTracker.GetWritingMode().IsBidiLTR();
+      const bool isAlignLeft = (mAlignContent == NS_STYLE_ALIGN_LEFT);
+      mAlignContent = (isAlignLeft == isLTR) ? NS_STYLE_ALIGN_START
+                                             : NS_STYLE_ALIGN_END;
+    }
+  }
+
+  // Map 'start'/'end' to 'flex-start'/'flex-end'.
+  if (mAlignContent == NS_STYLE_ALIGN_START) {
+    mAlignContent = NS_STYLE_ALIGN_FLEX_START;
+  } else if (mAlignContent == NS_STYLE_ALIGN_END) {
+    mAlignContent = NS_STYLE_ALIGN_FLEX_END;
+  }
+
+  // If our cross axis is (internally) reversed, swap the align-content
+  // "flex-start" and "flex-end" behaviors:
+  if (aAxisTracker.AreAxesInternallyReversed()) {
+    if (mAlignContent == NS_STYLE_ALIGN_FLEX_START) {
+      mAlignContent = NS_STYLE_ALIGN_FLEX_END;
+    } else if (mAlignContent == NS_STYLE_ALIGN_FLEX_END) {
+      mAlignContent = NS_STYLE_ALIGN_FLEX_START;
+    }
+  }
+
+  // Figure out how much space we'll set aside for packing spaces, and advance
+  // past any leading packing-space.
+  if (mPackingSpaceRemaining != 0) {
+    switch (mAlignContent) {
+      case NS_STYLE_ALIGN_SELF_START:
+      case NS_STYLE_ALIGN_SELF_END:
+      case NS_STYLE_ALIGN_BASELINE:
+      case NS_STYLE_ALIGN_LAST_BASELINE:
+        NS_WARNING("NYI: align-items/align-self:left/right/self-start/self-end/baseline/last baseline");
+        MOZ_FALLTHROUGH;
+      case NS_STYLE_ALIGN_FLEX_START:
+        // All packing space should go at the end --> nothing to do here.
+        break;
+      case NS_STYLE_ALIGN_FLEX_END:
+        // All packing space goes at the beginning
+        mPosition += mPackingSpaceRemaining;
+        break;
+      case NS_STYLE_ALIGN_CENTER:
+        // Half the packing space goes at the beginning
+        mPosition += mPackingSpaceRemaining / 2;
+        break;
+      case NS_STYLE_ALIGN_SPACE_BETWEEN:
+      case NS_STYLE_ALIGN_SPACE_AROUND:
+      case NS_STYLE_ALIGN_SPACE_EVENLY:
+        nsFlexContainerFrame::CalculatePackingSpace(numLines,
+                                                    mAlignContent,
+                                                    &mPosition,
+                                                    &mNumPackingSpacesRemaining,
+                                                    &mPackingSpaceRemaining);
+        break;
+      case NS_STYLE_ALIGN_STRETCH: {
+        // Split space equally between the lines:
+        MOZ_ASSERT(mPackingSpaceRemaining > 0,
+                   "negative packing space should make us use 'flex-start' "
+                   "instead of 'stretch' (and we shouldn't bother with this "
+                   "code if we have 0 packing space)");
+
+        uint32_t numLinesLeft = numLines;
+        for (FlexLine* line = aFirstLine; line; line = line->getNext()) {
+          // Our share is the amount of space remaining, divided by the number
+          // of lines remainig.
+          MOZ_ASSERT(numLinesLeft > 0, "miscalculated num lines");
+          nscoord shareOfExtraSpace = mPackingSpaceRemaining / numLinesLeft;
+          nscoord newSize = line->GetLineCrossSize() + shareOfExtraSpace;
+          line->SetLineCrossSize(newSize);
+
+          mPackingSpaceRemaining -= shareOfExtraSpace;
+          numLinesLeft--;
+        }
+        MOZ_ASSERT(numLinesLeft == 0, "miscalculated num lines");
+        break;
+      }
+      default:
+        MOZ_ASSERT_UNREACHABLE("Unexpected align-content value");
+    }
+  }
+}
+
+void
+CrossAxisPositionTracker::TraversePackingSpace()
+{
+  if (mNumPackingSpacesRemaining) {
+    MOZ_ASSERT(mAlignContent == NS_STYLE_ALIGN_SPACE_BETWEEN ||
+               mAlignContent == NS_STYLE_ALIGN_SPACE_AROUND ||
+               mAlignContent == NS_STYLE_ALIGN_SPACE_EVENLY,
+               "mNumPackingSpacesRemaining only applies for "
+               "space-between/space-around/space-evenly");
+
+    MOZ_ASSERT(mPackingSpaceRemaining >= 0,
+               "ran out of packing space earlier than we expected");
+
+    // NOTE: This integer math will skew the distribution of remainder
+    // app-units towards the end, which is fine.
+    nscoord curPackingSpace =
+      mPackingSpaceRemaining / mNumPackingSpacesRemaining;
+
+    mPosition += curPackingSpace;
+    mNumPackingSpacesRemaining--;
+    mPackingSpaceRemaining -= curPackingSpace;
+  }
+}
+
+SingleLineCrossAxisPositionTracker::
+  SingleLineCrossAxisPositionTracker(const FlexboxAxisTracker& aAxisTracker)
+  : PositionTracker(aAxisTracker.GetCrossAxis(),
+                    aAxisTracker.IsCrossAxisReversed())
+{
+}
+
+void
+FlexLine::ComputeCrossSizeAndBaseline(const FlexboxAxisTracker& aAxisTracker)
+{
+  nscoord crossStartToFurthestFirstBaseline = nscoord_MIN;
+  nscoord crossEndToFurthestFirstBaseline = nscoord_MIN;
+  nscoord crossStartToFurthestLastBaseline = nscoord_MIN;
+  nscoord crossEndToFurthestLastBaseline = nscoord_MIN;
+  nscoord largestOuterCrossSize = 0;
+  for (const FlexItem* item = mItems.getFirst(); item; item = item->getNext()) {
+    nscoord curOuterCrossSize =
+      item->GetOuterCrossSize(aAxisTracker.GetCrossAxis());
+
+    if ((item->GetAlignSelf() == NS_STYLE_ALIGN_BASELINE ||
+        item->GetAlignSelf() == NS_STYLE_ALIGN_LAST_BASELINE) &&
+        item->GetNumAutoMarginsInAxis(aAxisTracker.GetCrossAxis()) == 0) {
+      const bool useFirst = (item->GetAlignSelf() == NS_STYLE_ALIGN_BASELINE);
+      // FIXME: Once we support "writing-mode", we'll have to do baseline
+      // alignment in vertical flex containers here (w/ horizontal cross-axes).
+
+      // Find distance from our item's cross-start and cross-end margin-box
+      // edges to its baseline.
+      //
+      // Here's a diagram of a flex-item that we might be doing this on.
+      // "mmm" is the margin-box, "bbb" is the border-box. The bottom of
+      // the text "BASE" is the baseline.
+      //
+      // ---(cross-start)---
+      //                ___              ___            ___
+      //   mmmmmmmmmmmm  |                |margin-start  |
+      //   m          m  |               _|_   ___       |
+      //   m bbbbbbbb m  |curOuterCrossSize     |        |crossStartToBaseline
+      //   m b      b m  |                      |ascent  |
+      //   m b BASE b m  |                     _|_      _|_
+      //   m b      b m  |                               |
+      //   m bbbbbbbb m  |                               |crossEndToBaseline
+      //   m          m  |                               |
+      //   mmmmmmmmmmmm _|_                             _|_
+      //
+      // ---(cross-end)---
+      //
+      // We already have the curOuterCrossSize, margin-start, and the ascent.
+      // * We can get crossStartToBaseline by adding margin-start + ascent.
+      // * If we subtract that from the curOuterCrossSize, we get
+      //   crossEndToBaseline.
+
+      nscoord crossStartToBaseline =
+        item->GetBaselineOffsetFromOuterCrossEdge(eAxisEdge_Start,
+                                                  aAxisTracker,
+                                                  useFirst);
+      nscoord crossEndToBaseline = curOuterCrossSize - crossStartToBaseline;
+
+      // Now, update our "largest" values for these (across all the flex items
+      // in this flex line), so we can use them in computing the line's cross
+      // size below:
+      if (useFirst) {
+        crossStartToFurthestFirstBaseline =
+          std::max(crossStartToFurthestFirstBaseline, crossStartToBaseline);
+        crossEndToFurthestFirstBaseline =
+          std::max(crossEndToFurthestFirstBaseline, crossEndToBaseline);
+      } else {
+        crossStartToFurthestLastBaseline =
+          std::max(crossStartToFurthestLastBaseline, crossStartToBaseline);
+        crossEndToFurthestLastBaseline =
+          std::max(crossEndToFurthestLastBaseline, crossEndToBaseline);
+      }
+    } else {
+      largestOuterCrossSize = std::max(largestOuterCrossSize, curOuterCrossSize);
+    }
+  }
+
+  // The line's baseline offset is the distance from the line's edge (start or
+  // end, depending on whether we've flipped the axes) to the furthest
+  // item-baseline. The item(s) with that baseline will be exactly aligned with
+  // the line's edge.
+  mFirstBaselineOffset = aAxisTracker.AreAxesInternallyReversed() ?
+    crossEndToFurthestFirstBaseline : crossStartToFurthestFirstBaseline;
+
+  mLastBaselineOffset = aAxisTracker.AreAxesInternallyReversed() ?
+    crossStartToFurthestLastBaseline : crossEndToFurthestLastBaseline;
+
+  // The line's cross-size is the larger of:
+  //  (a) [largest cross-start-to-baseline + largest baseline-to-cross-end] of
+  //      all baseline-aligned items with no cross-axis auto margins...
+  // and
+  //  (b) [largest cross-start-to-baseline + largest baseline-to-cross-end] of
+  //      all last baseline-aligned items with no cross-axis auto margins...
+  // and
+  //  (c) largest cross-size of all other children.
+  mLineCrossSize = std::max(
+    std::max(crossStartToFurthestFirstBaseline + crossEndToFurthestFirstBaseline,
+             crossStartToFurthestLastBaseline + crossEndToFurthestLastBaseline),
+    largestOuterCrossSize);
+}
+
+void
+FlexItem::ResolveStretchedCrossSize(nscoord aLineCrossSize,
+                                    const FlexboxAxisTracker& aAxisTracker)
+{
+  AxisOrientationType crossAxis = aAxisTracker.GetCrossAxis();
+  // We stretch IFF we are align-self:stretch, have no auto margins in
+  // cross axis, and have cross-axis size property == "auto". If any of those
+  // conditions don't hold up, we won't stretch.
+  if (mAlignSelf != NS_STYLE_ALIGN_STRETCH ||
+      GetNumAutoMarginsInAxis(crossAxis) != 0 ||
+      eStyleUnit_Auto != aAxisTracker.ComputedCrossSize(mFrame).GetUnit()) {
+    return;
+  }
+
+  // If we've already been stretched, we can bail out early, too.
+  // No need to redo the calculation.
+  if (mIsStretched) {
+    return;
+  }
+
+  // Reserve space for margins & border & padding, and then use whatever
+  // remains as our item's cross-size (clamped to its min/max range).
+  nscoord stretchedSize = aLineCrossSize -
+    GetMarginBorderPaddingSizeInAxis(crossAxis);
+
+  stretchedSize = NS_CSS_MINMAX(stretchedSize, mCrossMinSize, mCrossMaxSize);
+
+  // Update the cross-size & make a note that it's stretched, so we know to
+  // override the reflow state's computed cross-size in our final reflow.
+  SetCrossSize(stretchedSize);
+  mIsStretched = true;
+}
+
+void
+SingleLineCrossAxisPositionTracker::
+  ResolveAutoMarginsInCrossAxis(const FlexLine& aLine,
+                                FlexItem& aItem)
+{
+  // Subtract the space that our item is already occupying, to see how much
+  // space (if any) is available for its auto margins.
+  nscoord spaceForAutoMargins = aLine.GetLineCrossSize() -
+    aItem.GetOuterCrossSize(mAxis);
+
+  if (spaceForAutoMargins <= 0) {
+    return; // No available space  --> nothing to do
+  }
+
+  uint32_t numAutoMargins = aItem.GetNumAutoMarginsInAxis(mAxis);
+  if (numAutoMargins == 0) {
+    return; // No auto margins --> nothing to do.
+  }
+
+  // OK, we have at least one auto margin and we have some available space.
+  // Give each auto margin a share of the space.
+  const nsStyleSides& styleMargin = aItem.Frame()->StyleMargin()->mMargin;
+  for (uint32_t i = 0; i < eNumAxisEdges; i++) {
+    mozilla::Side side = kAxisOrientationToSidesMap[mAxis][i];
+    if (styleMargin.GetUnit(side) == eStyleUnit_Auto) {
+      MOZ_ASSERT(aItem.GetMarginComponentForSide(side) == 0,
+                 "Expecting auto margins to have value '0' before we "
+                 "update them");
+
+      // NOTE: integer divison is fine here; numAutoMargins is either 1 or 2.
+      // If it's 2 & spaceForAutoMargins is odd, 1st margin gets smaller half.
+      nscoord curAutoMarginSize = spaceForAutoMargins / numAutoMargins;
+      aItem.SetMarginComponentForSide(side, curAutoMarginSize);
+      numAutoMargins--;
+      spaceForAutoMargins -= curAutoMarginSize;
+    }
+  }
+}
+
+void
+SingleLineCrossAxisPositionTracker::
+  EnterAlignPackingSpace(const FlexLine& aLine,
+                         const FlexItem& aItem,
+                         const FlexboxAxisTracker& aAxisTracker)
+{
+  // We don't do align-self alignment on items that have auto margins
+  // in the cross axis.
+  if (aItem.GetNumAutoMarginsInAxis(mAxis)) {
+    return;
+  }
+
+  uint8_t alignSelf = aItem.GetAlignSelf();
+  // NOTE: 'stretch' behaves like 'flex-start' once we've stretched any
+  // auto-sized items (which we've already done).
+  if (alignSelf == NS_STYLE_ALIGN_STRETCH) {
+    alignSelf = NS_STYLE_ALIGN_FLEX_START;
+  }
+
+  // Map 'left'/'right' to 'start'/'end'
+  if (alignSelf == NS_STYLE_ALIGN_LEFT || alignSelf == NS_STYLE_ALIGN_RIGHT) {
+    if (aAxisTracker.IsRowOriented()) {
+      // Container's alignment axis is not parallel to the inline axis,
+      // so we map both 'left' and 'right' to 'start'.
+      alignSelf = NS_STYLE_ALIGN_START;
+    } else {
+      // Column-oriented, so we map 'left' and 'right' to 'start' or 'end',
+      // depending on left-to-right writing mode.
+      const bool isLTR = aAxisTracker.GetWritingMode().IsBidiLTR();
+      const bool isAlignLeft = (alignSelf == NS_STYLE_ALIGN_LEFT);
+      alignSelf = (isAlignLeft == isLTR) ? NS_STYLE_ALIGN_START
+                                         : NS_STYLE_ALIGN_END;
+    }
+  }
+
+  // Map 'start'/'end' to 'flex-start'/'flex-end'.
+  if (alignSelf == NS_STYLE_ALIGN_START) {
+    alignSelf = NS_STYLE_ALIGN_FLEX_START;
+  } else if (alignSelf == NS_STYLE_ALIGN_END) {
+    alignSelf = NS_STYLE_ALIGN_FLEX_END;
+  }
+
+  // If our cross axis is (internally) reversed, swap the align-self
+  // "flex-start" and "flex-end" behaviors:
+  if (aAxisTracker.AreAxesInternallyReversed()) {
+    if (alignSelf == NS_STYLE_ALIGN_FLEX_START) {
+      alignSelf = NS_STYLE_ALIGN_FLEX_END;
+    } else if (alignSelf == NS_STYLE_ALIGN_FLEX_END) {
+      alignSelf = NS_STYLE_ALIGN_FLEX_START;
+    }
+  }
+
+  switch (alignSelf) {
+    case NS_STYLE_ALIGN_SELF_START:
+    case NS_STYLE_ALIGN_SELF_END:
+      NS_WARNING("NYI: align-items/align-self:left/right/self-start/self-end");
+      MOZ_FALLTHROUGH;
+    case NS_STYLE_ALIGN_FLEX_START:
+      // No space to skip over -- we're done.
+      break;
+    case NS_STYLE_ALIGN_FLEX_END:
+      mPosition += aLine.GetLineCrossSize() - aItem.GetOuterCrossSize(mAxis);
+      break;
+    case NS_STYLE_ALIGN_CENTER:
+      // Note: If cross-size is odd, the "after" space will get the extra unit.
+      mPosition +=
+        (aLine.GetLineCrossSize() - aItem.GetOuterCrossSize(mAxis)) / 2;
+      break;
+    case NS_STYLE_ALIGN_BASELINE:
+    case NS_STYLE_ALIGN_LAST_BASELINE: {
+      const bool useFirst = (alignSelf == NS_STYLE_ALIGN_BASELINE);
+
+      // Normally, baseline-aligned items are collectively aligned with the
+      // line's cross-start edge; however, if our cross axis is (internally)
+      // reversed, we instead align them with the cross-end edge.
+      // A similar logic holds for last baseline-aligned items, but in reverse.
+      AxisEdgeType baselineAlignEdge =
+        aAxisTracker.AreAxesInternallyReversed() == useFirst ?
+        eAxisEdge_End : eAxisEdge_Start;
+
+      nscoord itemBaselineOffset =
+        aItem.GetBaselineOffsetFromOuterCrossEdge(baselineAlignEdge,
+                                                  aAxisTracker,
+                                                  useFirst);
+
+      nscoord lineBaselineOffset = useFirst ? aLine.GetFirstBaselineOffset()
+                                            : aLine.GetLastBaselineOffset();
+
+      NS_ASSERTION(lineBaselineOffset >= itemBaselineOffset,
+                   "failed at finding largest baseline offset");
+
+      // How much do we need to adjust our position (from the line edge),
+      // to get the item's baseline to hit the line's baseline offset:
+      nscoord baselineDiff = lineBaselineOffset - itemBaselineOffset;
+
+      if (aAxisTracker.AreAxesInternallyReversed() == useFirst) {
+        // Advance to align item w/ line's flex-end edge (as in FLEX_END case):
+        mPosition += aLine.GetLineCrossSize() - aItem.GetOuterCrossSize(mAxis);
+        // ...and step *back* by the baseline adjustment:
+        mPosition -= baselineDiff;
+      } else {
+        // mPosition is already at line's flex-start edge.
+        // From there, we step *forward* by the baseline adjustment:
+        mPosition += baselineDiff;
+      }
+      break;
+    }
+    default:
+      MOZ_ASSERT_UNREACHABLE("Unexpected align-self value");
+      break;
+  }
+}
+
+// Utility function to convert an InlineDir to an AxisOrientationType
+static inline AxisOrientationType
+InlineDirToAxisOrientation(WritingMode::InlineDir aInlineDir)
+{
+  switch (aInlineDir) {
+    case WritingMode::eInlineLTR:
+      return eAxis_LR;
+    case WritingMode::eInlineRTL:
+      return eAxis_RL;
+    case WritingMode::eInlineTTB:
+      return eAxis_TB;
+    case WritingMode::eInlineBTT:
+      return eAxis_BT;
+  }
+
+  MOZ_ASSERT_UNREACHABLE("Unhandled InlineDir");
+  return eAxis_LR; // in case of unforseen error, assume English LTR text flow.
+}
+
+// Utility function to convert a BlockDir to an AxisOrientationType
+static inline AxisOrientationType
+BlockDirToAxisOrientation(WritingMode::BlockDir aBlockDir)
+{
+  switch (aBlockDir) {
+    case WritingMode::eBlockLR:
+      return eAxis_LR;
+    case WritingMode::eBlockRL:
+      return eAxis_RL;
+    case WritingMode::eBlockTB:
+      return eAxis_TB;
+    // NOTE: WritingMode::eBlockBT (bottom-to-top) does not exist.
+  }
+
+  MOZ_ASSERT_UNREACHABLE("Unhandled BlockDir");
+  return eAxis_TB; // in case of unforseen error, assume English TTB block-flow
+}
+
+FlexboxAxisTracker::FlexboxAxisTracker(
+  const nsFlexContainerFrame* aFlexContainer,
+  const WritingMode& aWM,
+  AxisTrackerFlags aFlags)
+  : mWM(aWM),
+    mAreAxesInternallyReversed(false)
+{
+  if (IsLegacyBox(aFlexContainer)) {
+    InitAxesFromLegacyProps(aFlexContainer);
+  } else {
+    InitAxesFromModernProps(aFlexContainer);
+  }
+
+  // Master switch to enable/disable bug 983427's code for reversing our axes
+  // and reversing some logic, to avoid reflowing children in bottom-to-top
+  // order. (This switch can be removed eventually, but for now, it allows
+  // this special-case code path to be compared against the normal code path.)
+  static bool sPreventBottomToTopChildOrdering = true;
+
+  // Note: if the eAllowBottomToTopChildOrdering flag is set, that overrides
+  // the static boolean and makes us skip this special case.
+  if (!(aFlags & AxisTrackerFlags::eAllowBottomToTopChildOrdering) &&
+      sPreventBottomToTopChildOrdering) {
+    // If either axis is bottom-to-top, we flip both axes (and set a flag
+    // so that we can flip some logic to make the reversal transparent).
+    if (eAxis_BT == mMainAxis || eAxis_BT == mCrossAxis) {
+      mMainAxis = GetReverseAxis(mMainAxis);
+      mCrossAxis = GetReverseAxis(mCrossAxis);
+      mAreAxesInternallyReversed = true;
+      mIsMainAxisReversed = !mIsMainAxisReversed;
+      mIsCrossAxisReversed = !mIsCrossAxisReversed;
+    }
+  }
+}
+
+void
+FlexboxAxisTracker::InitAxesFromLegacyProps(
+  const nsFlexContainerFrame* aFlexContainer)
+{
+  const nsStyleXUL* styleXUL = aFlexContainer->StyleXUL();
+
+  const bool boxOrientIsVertical = (styleXUL->mBoxOrient ==
+                                    StyleBoxOrient::Vertical);
+  const bool wmIsVertical = mWM.IsVertical();
+
+  // If box-orient agrees with our writing-mode, then we're "row-oriented"
+  // (i.e. the flexbox main axis is the same as our writing mode's inline
+  // direction).  Otherwise, we're column-oriented (i.e. the flexbox's main
+  // axis is perpendicular to the writing-mode's inline direction).
+  mIsRowOriented = (boxOrientIsVertical == wmIsVertical);
+
+  // XXXdholbert BEGIN CODE TO SET DEPRECATED MEMBER-VARS
+  if (boxOrientIsVertical) {
+    mMainAxis = eAxis_TB;
+    mCrossAxis = eAxis_LR;
+  } else {
+    mMainAxis = eAxis_LR;
+    mCrossAxis = eAxis_TB;
+  }
+  // "direction: rtl" reverses the writing-mode's inline axis.
+  // So, we need to reverse the corresponding flex axis to match.
+  // (Note this we don't toggle "mIsMainAxisReversed" for this condition,
+  // because the main axis will still match mWM's inline direction.)
+  if (!mWM.IsBidiLTR()) {
+    AxisOrientationType& axisToFlip = mIsRowOriented ? mMainAxis : mCrossAxis;
+    axisToFlip = GetReverseAxis(axisToFlip);
+  }
+  // XXXdholbert END CODE TO SET DEPRECATED MEMBER-VARS
+
+  // Legacy flexbox can use "-webkit-box-direction: reverse" to reverse the
+  // main axis (so it runs in the reverse direction of the inline axis):
+  if (styleXUL->mBoxDirection == StyleBoxDirection::Reverse) {
+    mMainAxis = GetReverseAxis(mMainAxis);
+    mIsMainAxisReversed = true;
+  } else {
+    mIsMainAxisReversed = false;
+  }
+
+  // Legacy flexbox does not support reversing the cross axis -- it has no
+  // equivalent of modern flexbox's "flex-wrap: wrap-reverse".
+  mIsCrossAxisReversed = false;
+}
+
+void
+FlexboxAxisTracker::InitAxesFromModernProps(
+  const nsFlexContainerFrame* aFlexContainer)
+{
+  const nsStylePosition* stylePos = aFlexContainer->StylePosition();
+  uint32_t flexDirection = stylePos->mFlexDirection;
+
+  // Inline dimension ("start-to-end"):
+  // (NOTE: I'm intentionally not calling these "inlineAxis"/"blockAxis", since
+  // those terms have explicit definition in the writing-modes spec, which are
+  // the opposite of how I'd be using them here.)
+  AxisOrientationType inlineDimension =
+    InlineDirToAxisOrientation(mWM.GetInlineDir());
+  AxisOrientationType blockDimension =
+    BlockDirToAxisOrientation(mWM.GetBlockDir());
+
+  // Determine main axis:
+  switch (flexDirection) {
+    case NS_STYLE_FLEX_DIRECTION_ROW:
+      mMainAxis = inlineDimension;
+      mIsRowOriented = true;
+      mIsMainAxisReversed = false;
+      break;
+    case NS_STYLE_FLEX_DIRECTION_ROW_REVERSE:
+      mMainAxis = GetReverseAxis(inlineDimension);
+      mIsRowOriented = true;
+      mIsMainAxisReversed = true;
+      break;
+    case NS_STYLE_FLEX_DIRECTION_COLUMN:
+      mMainAxis = blockDimension;
+      mIsRowOriented = false;
+      mIsMainAxisReversed = false;
+      break;
+    case NS_STYLE_FLEX_DIRECTION_COLUMN_REVERSE:
+      mMainAxis = GetReverseAxis(blockDimension);
+      mIsRowOriented = false;
+      mIsMainAxisReversed = true;
+      break;
+    default:
+      MOZ_ASSERT_UNREACHABLE("Unexpected flex-direction value");
+  }
+
+  // Determine cross axis:
+  // (This is set up so that a bogus |flexDirection| value will
+  // give us blockDimension.
+  if (flexDirection == NS_STYLE_FLEX_DIRECTION_COLUMN ||
+      flexDirection == NS_STYLE_FLEX_DIRECTION_COLUMN_REVERSE) {
+    mCrossAxis = inlineDimension;
+  } else {
+    mCrossAxis = blockDimension;
+  }
+
+  // "flex-wrap: wrap-reverse" reverses our cross axis.
+  if (stylePos->mFlexWrap == NS_STYLE_FLEX_WRAP_WRAP_REVERSE) {
+    mCrossAxis = GetReverseAxis(mCrossAxis);
+    mIsCrossAxisReversed = true;
+  } else {
+    mIsCrossAxisReversed = false;
+  }
+}
+
+// Allocates a new FlexLine, adds it to the given LinkedList (at the front or
+// back depending on aShouldInsertAtFront), and returns a pointer to it.
+static FlexLine*
+AddNewFlexLineToList(LinkedList<FlexLine>& aLines,
+                     bool aShouldInsertAtFront)
+{
+  FlexLine* newLine = new FlexLine();
+  if (aShouldInsertAtFront) {
+    aLines.insertFront(newLine);
+  } else {
+    aLines.insertBack(newLine);
+  }
+  return newLine;
+}
+
+void
+nsFlexContainerFrame::GenerateFlexLines(
+  nsPresContext* aPresContext,
+  const ReflowInput& aReflowInput,
+  nscoord aContentBoxMainSize,
+  nscoord aAvailableBSizeForContent,
+  const nsTArray<StrutInfo>& aStruts,
+  const FlexboxAxisTracker& aAxisTracker,
+  nsTArray<nsIFrame*>& aPlaceholders, /* out */
+  LinkedList<FlexLine>& aLines /* out */)
+{
+  MOZ_ASSERT(aLines.isEmpty(), "Expecting outparam to start out empty");
+
+  const bool isSingleLine =
+    NS_STYLE_FLEX_WRAP_NOWRAP == aReflowInput.mStylePosition->mFlexWrap;
+
+  // If we're transparently reversing axes, then we'll need to link up our
+  // FlexItems and FlexLines in the reverse order, so that the rest of flex
+  // layout (with flipped axes) will still produce the correct result.
+  // Here, we declare a convenience bool that we'll pass when adding a new
+  // FlexLine or FlexItem, to make us insert it at the beginning of its list
+  // (so the list ends up reversed).
+  const bool shouldInsertAtFront = aAxisTracker.AreAxesInternallyReversed();
+
+  // We have at least one FlexLine. Even an empty flex container has a single
+  // (empty) flex line.
+  FlexLine* curLine = AddNewFlexLineToList(aLines, shouldInsertAtFront);
+
+  nscoord wrapThreshold;
+  if (isSingleLine) {
+    // Not wrapping. Set threshold to sentinel value that tells us not to wrap.
+    wrapThreshold = NS_UNCONSTRAINEDSIZE;
+  } else {
+    // Wrapping! Set wrap threshold to flex container's content-box main-size.
+    wrapThreshold = aContentBoxMainSize;
+
+    // If the flex container doesn't have a definite content-box main-size
+    // (e.g. if main axis is vertical & 'height' is 'auto'), make sure we at
+    // least wrap when we hit its max main-size.
+    if (wrapThreshold == NS_UNCONSTRAINEDSIZE) {
+      const nscoord flexContainerMaxMainSize =
+        GET_MAIN_COMPONENT_LOGICAL(aAxisTracker, aAxisTracker.GetWritingMode(),
+                                   aReflowInput.ComputedMaxISize(),
+                                   aReflowInput.ComputedMaxBSize());
+
+      wrapThreshold = flexContainerMaxMainSize;
+    }
+
+    // Also: if we're column-oriented and paginating in the block dimension,
+    // we may need to wrap to a new flex line sooner (before we grow past the
+    // available BSize, potentially running off the end of the page).
+    if (aAxisTracker.IsColumnOriented() &&
+        aAvailableBSizeForContent != NS_UNCONSTRAINEDSIZE) {
+      wrapThreshold = std::min(wrapThreshold, aAvailableBSizeForContent);
+    }
+  }
+
+  // Tracks the index of the next strut, in aStruts (and when this hits
+  // aStruts.Length(), that means there are no more struts):
+  uint32_t nextStrutIdx = 0;
+
+  // Overall index of the current flex item in the flex container. (This gets
+  // checked against entries in aStruts.)
+  uint32_t itemIdxInContainer = 0;
+
+  for (nsIFrame* childFrame : mFrames) {
+    // Don't create flex items / lines for placeholder frames:
+    if (childFrame->GetType() == nsGkAtoms::placeholderFrame) {
+      aPlaceholders.AppendElement(childFrame);
+      continue;
+    }
+
+    // Honor "page-break-before", if we're multi-line and this line isn't empty:
+    if (!isSingleLine && !curLine->IsEmpty() &&
+        childFrame->StyleDisplay()->mBreakBefore) {
+      curLine = AddNewFlexLineToList(aLines, shouldInsertAtFront);
+    }
+
+    UniquePtr<FlexItem> item;
+    if (nextStrutIdx < aStruts.Length() &&
+        aStruts[nextStrutIdx].mItemIdx == itemIdxInContainer) {
+
+      // Use the simplified "strut" FlexItem constructor:
+      item = MakeUnique<FlexItem>(childFrame, aStruts[nextStrutIdx].mStrutCrossSize,
+                                  aReflowInput.GetWritingMode());
+      nextStrutIdx++;
+    } else {
+      item = GenerateFlexItemForChild(aPresContext, childFrame,
+                                      aReflowInput, aAxisTracker);
+    }
+
+    nscoord itemInnerHypotheticalMainSize = item->GetMainSize();
+    nscoord itemOuterHypotheticalMainSize =
+      item->GetOuterMainSize(aAxisTracker.GetMainAxis());
+
+    // Check if we need to wrap |item| to a new line
+    // (i.e. check if its outer hypothetical main size pushes our line over
+    // the threshold)
+    if (wrapThreshold != NS_UNCONSTRAINEDSIZE &&
+        !curLine->IsEmpty() && // No need to wrap at start of a line.
+        wrapThreshold < (curLine->GetTotalOuterHypotheticalMainSize() +
+                         itemOuterHypotheticalMainSize)) {
+      curLine = AddNewFlexLineToList(aLines, shouldInsertAtFront);
+    }
+
+    // Add item to current flex line (and update the line's bookkeeping about
+    // how large its items collectively are).
+    curLine->AddItem(item.release(), shouldInsertAtFront,
+                     itemInnerHypotheticalMainSize,
+                     itemOuterHypotheticalMainSize);
+
+    // Honor "page-break-after", if we're multi-line and have more children:
+    if (!isSingleLine && childFrame->GetNextSibling() &&
+        childFrame->StyleDisplay()->mBreakAfter) {
+      curLine = AddNewFlexLineToList(aLines, shouldInsertAtFront);
+    }
+    itemIdxInContainer++;
+  }
+}
+
+// Retrieves the content-box main-size of our flex container from the
+// reflow state (specifically, the main-size of *this continuation* of the
+// flex container).
+nscoord
+nsFlexContainerFrame::GetMainSizeFromReflowInput(
+  const ReflowInput& aReflowInput,
+  const FlexboxAxisTracker& aAxisTracker)
+{
+  if (aAxisTracker.IsRowOriented()) {
+    // Row-oriented --> our main axis is the inline axis, so our main size
+    // is our inline size (which should already be resolved).
+    NS_WARNING_ASSERTION(
+      aReflowInput.ComputedISize() != NS_UNCONSTRAINEDSIZE,
+      "Unconstrained inline size; this should only result from huge sizes "
+      "(not intrinsic sizing w/ orthogonal flows)");
+    return aReflowInput.ComputedISize();
+  }
+
+  // Note: This may be unconstrained, if our block size is "auto":
+  return GetEffectiveComputedBSize(aReflowInput);
+}
+
+// Returns the largest outer hypothetical main-size of any line in |aLines|.
+// (i.e. the hypothetical main-size of the largest line)
+static nscoord
+GetLargestLineMainSize(const FlexLine* aFirstLine)
+{
+  nscoord largestLineOuterSize = 0;
+  for (const FlexLine* line = aFirstLine; line; line = line->getNext()) {
+    largestLineOuterSize = std::max(largestLineOuterSize,
+                                    line->GetTotalOuterHypotheticalMainSize());
+  }
+  return largestLineOuterSize;
+}
+
+/* Resolves the content-box main-size of a flex container frame,
+ * primarily based on:
+ * - the "tentative" main size, taken from the reflow state ("tentative"
+ *    because it may be unconstrained or may run off the page).
+ * - the available BSize (needed if the main axis is the block axis).
+ * - the sizes of our lines of flex items.
+ *
+ * Guaranteed to return a definite length, i.e. not NS_UNCONSTRAINEDSIZE,
+ * aside from cases with huge lengths which happen to compute to that value.
+ *
+ * (Note: This function should be structurally similar to 'ComputeCrossSize()',
+ * except that here, the caller has already grabbed the tentative size from the
+ * reflow state.)
+ */
+static nscoord
+ResolveFlexContainerMainSize(const ReflowInput& aReflowInput,
+                             const FlexboxAxisTracker& aAxisTracker,
+                             nscoord aTentativeMainSize,
+                             nscoord aAvailableBSizeForContent,
+                             const FlexLine* aFirstLine,
+                             nsReflowStatus& aStatus)
+{
+  MOZ_ASSERT(aFirstLine, "null first line pointer");
+
+  if (aAxisTracker.IsRowOriented()) {
+    // Row-oriented --> our main axis is the inline axis, so our main size
+    // is our inline size (which should already be resolved).
+    return aTentativeMainSize;
+  }
+
+  if (aTentativeMainSize != NS_INTRINSICSIZE) {
+    // Column-oriented case, with fixed BSize:
+    if (aAvailableBSizeForContent == NS_UNCONSTRAINEDSIZE ||
+        aTentativeMainSize < aAvailableBSizeForContent) {
+      // Not in a fragmenting context, OR no need to fragment because we have
+      // more available BSize than we need. Either way, we don't need to clamp.
+      // (Note that the reflow state has already done the appropriate
+      // min/max-BSize clamping.)
+      return aTentativeMainSize;
+    }
+
+    // Fragmenting *and* our fixed BSize is larger than available BSize:
+    // Mark incomplete so we get a next-in-flow, and take up all of the
+    // available BSize (or the amount of BSize required by our children, if
+    // that's larger; but of course not more than our own computed BSize).
+    // XXXdholbert For now, we don't support pushing children to our next
+    // continuation or splitting children, so "amount of BSize required by
+    // our children" is just the main-size (BSize) of our longest flex line.
+    NS_FRAME_SET_INCOMPLETE(aStatus);
+    nscoord largestLineOuterSize = GetLargestLineMainSize(aFirstLine);
+
+    if (largestLineOuterSize <= aAvailableBSizeForContent) {
+      return aAvailableBSizeForContent;
+    }
+    return std::min(aTentativeMainSize, largestLineOuterSize);
+  }
+
+  // Column-oriented case, with auto BSize:
+  // Resolve auto BSize to the largest FlexLine length, clamped to our
+  // computed min/max main-size properties.
+  // XXXdholbert Handle constrained-aAvailableBSizeForContent case here.
+  nscoord largestLineOuterSize = GetLargestLineMainSize(aFirstLine);
+  return NS_CSS_MINMAX(largestLineOuterSize,
+                       aReflowInput.ComputedMinBSize(),
+                       aReflowInput.ComputedMaxBSize());
+}
+
+nscoord
+nsFlexContainerFrame::ComputeCrossSize(const ReflowInput& aReflowInput,
+                                       const FlexboxAxisTracker& aAxisTracker,
+                                       nscoord aSumLineCrossSizes,
+                                       nscoord aAvailableBSizeForContent,
+                                       bool* aIsDefinite,
+                                       nsReflowStatus& aStatus)
+{
+  MOZ_ASSERT(aIsDefinite, "outparam pointer must be non-null");
+
+  if (aAxisTracker.IsColumnOriented()) {
+    // Column-oriented --> our cross axis is the inline axis, so our cross size
+    // is our inline size (which should already be resolved).
+    NS_WARNING_ASSERTION(
+      aReflowInput.ComputedISize() != NS_UNCONSTRAINEDSIZE,
+      "Unconstrained inline size; this should only result from huge sizes "
+      "(not intrinsic sizing w/ orthogonal flows)");
+    *aIsDefinite = true;
+    return aReflowInput.ComputedISize();
+  }
+
+  nscoord effectiveComputedBSize = GetEffectiveComputedBSize(aReflowInput);
+  if (effectiveComputedBSize != NS_INTRINSICSIZE) {
+    // Row-oriented case (cross axis is block-axis), with fixed BSize:
+    *aIsDefinite = true;
+    if (aAvailableBSizeForContent == NS_UNCONSTRAINEDSIZE ||
+        effectiveComputedBSize < aAvailableBSizeForContent) {
+      // Not in a fragmenting context, OR no need to fragment because we have
+      // more available BSize than we need. Either way, just use our fixed
+      // BSize.  (Note that the reflow state has already done the appropriate
+      // min/max-BSize clamping.)
+      return effectiveComputedBSize;
+    }
+
+    // Fragmenting *and* our fixed BSize is too tall for available BSize:
+    // Mark incomplete so we get a next-in-flow, and take up all of the
+    // available BSize (or the amount of BSize required by our children, if
+    // that's larger; but of course not more than our own computed BSize).
+    // XXXdholbert For now, we don't support pushing children to our next
+    // continuation or splitting children, so "amount of BSize required by
+    // our children" is just the sum of our FlexLines' BSizes (cross sizes).
+    NS_FRAME_SET_INCOMPLETE(aStatus);
+    if (aSumLineCrossSizes <= aAvailableBSizeForContent) {
+      return aAvailableBSizeForContent;
+    }
+    return std::min(effectiveComputedBSize, aSumLineCrossSizes);
+  }
+
+  // Row-oriented case (cross axis is block axis), with auto BSize:
+  // Shrink-wrap our line(s), subject to our min-size / max-size
+  // constraints in that (block) axis.
+  // XXXdholbert Handle constrained-aAvailableBSizeForContent case here.
+  *aIsDefinite = false;
+  return NS_CSS_MINMAX(aSumLineCrossSizes,
+                       aReflowInput.ComputedMinBSize(),
+                       aReflowInput.ComputedMaxBSize());
+}
+
+void
+FlexLine::PositionItemsInMainAxis(uint8_t aJustifyContent,
+                                  nscoord aContentBoxMainSize,
+                                  const FlexboxAxisTracker& aAxisTracker)
+{
+  MainAxisPositionTracker mainAxisPosnTracker(aAxisTracker, this,
+                                              aJustifyContent,
+                                              aContentBoxMainSize);
+  for (FlexItem* item = mItems.getFirst(); item; item = item->getNext()) {
+    nscoord itemMainBorderBoxSize =
+      item->GetMainSize() +
+      item->GetBorderPaddingSizeInAxis(mainAxisPosnTracker.GetAxis());
+
+    // Resolve any main-axis 'auto' margins on aChild to an actual value.
+    mainAxisPosnTracker.ResolveAutoMarginsInMainAxis(*item);
+
+    // Advance our position tracker to child's upper-left content-box corner,
+    // and use that as its position in the main axis.
+    mainAxisPosnTracker.EnterMargin(item->GetMargin());
+    mainAxisPosnTracker.EnterChildFrame(itemMainBorderBoxSize);
+
+    item->SetMainPosition(mainAxisPosnTracker.GetPosition());
+
+    mainAxisPosnTracker.ExitChildFrame(itemMainBorderBoxSize);
+    mainAxisPosnTracker.ExitMargin(item->GetMargin());
+    mainAxisPosnTracker.TraversePackingSpace();
+  }
+}
+
+/**
+ * Given the flex container's "flex-relative ascent" (i.e. distance from the
+ * flex container's content-box cross-start edge to its baseline), returns
+ * its actual physical ascent value (the distance from the *border-box* top
+ * edge to its baseline).
+ */
+static nscoord
+ComputePhysicalAscentFromFlexRelativeAscent(
+  nscoord aFlexRelativeAscent,
+  nscoord aContentBoxCrossSize,
+  const ReflowInput& aReflowInput,
+  const FlexboxAxisTracker& aAxisTracker)
+{
+  return aReflowInput.ComputedPhysicalBorderPadding().top +
+    PhysicalCoordFromFlexRelativeCoord(aFlexRelativeAscent,
+                                       aContentBoxCrossSize,
+                                       aAxisTracker.GetCrossAxis());
+}
+
+void
+nsFlexContainerFrame::SizeItemInCrossAxis(
+  nsPresContext* aPresContext,
+  const FlexboxAxisTracker& aAxisTracker,
+  ReflowInput& aChildReflowInput,
+  FlexItem& aItem)
+{
+  if (aAxisTracker.IsCrossAxisHorizontal()) {
+    MOZ_ASSERT(aItem.HasIntrinsicRatio(),
+               "For now, caller's CanMainSizeInfluenceCrossSize check should "
+               "only allow us to get here for items with intrinsic ratio");
+    // XXXdholbert When we finish support for vertical writing-modes,
+    // (in bug 1079155 or a dependency), we'll relax the horizontal check in
+    // CanMainSizeInfluenceCrossSize, and this function will need to be able
+    // to measure the baseline & width (given our resolved height)
+    // of vertical-writing-mode flex items here.
+    // For now, we only expect to get here for items with an intrinsic aspect
+    // ratio; and for those items, we can just read the size off of the reflow
+    // state, without performing reflow.
+    aItem.SetCrossSize(aChildReflowInput.ComputedWidth());
+    return;
+  }
+
+  MOZ_ASSERT(!aItem.HadMeasuringReflow(),
+             "We shouldn't need more than one measuring reflow");
+
+  if (aItem.GetAlignSelf() == NS_STYLE_ALIGN_STRETCH) {
+    // This item's got "align-self: stretch", so we probably imposed a
+    // stretched computed height on it during its previous reflow. We're
+    // not imposing that height for *this* measuring reflow, so we need to
+    // tell it to treat this reflow as a vertical resize (regardless of
+    // whether any of its ancestors are being resized).
+    aChildReflowInput.SetVResize(true);
+  }
+  ReflowOutput childDesiredSize(aChildReflowInput);
+  nsReflowStatus childReflowStatus;
+  const uint32_t flags = NS_FRAME_NO_MOVE_FRAME;
+  ReflowChild(aItem.Frame(), aPresContext,
+              childDesiredSize, aChildReflowInput,
+              0, 0, flags, childReflowStatus);
+  aItem.SetHadMeasuringReflow();
+
+  // XXXdholbert Once we do pagination / splitting, we'll need to actually
+  // handle incomplete childReflowStatuses. But for now, we give our kids
+  // unconstrained available height, which means they should always complete.
+  MOZ_ASSERT(NS_FRAME_IS_COMPLETE(childReflowStatus),
+             "We gave flex item unconstrained available height, so it "
+             "should be complete");
+
+  // Tell the child we're done with its initial reflow.
+  // (Necessary for e.g. GetBaseline() to work below w/out asserting)
+  FinishReflowChild(aItem.Frame(), aPresContext,
+                    childDesiredSize, &aChildReflowInput, 0, 0, flags);
+
+  // Save the sizing info that we learned from this reflow
+  // -----------------------------------------------------
+
+  // Tentatively store the child's desired content-box cross-size.
+  // Note that childDesiredSize is the border-box size, so we have to
+  // subtract border & padding to get the content-box size.
+  // (Note that at this point in the code, we know our cross axis is vertical,
+  // so we don't bother with making aAxisTracker pick the cross-axis component
+  // for us.)
+  nscoord crossAxisBorderPadding = aItem.GetBorderPadding().TopBottom();
+  if (childDesiredSize.Height() < crossAxisBorderPadding) {
+    // Child's requested size isn't large enough for its border/padding!
+    // This is OK for the trivial nsFrame::Reflow() impl, but other frame
+    // classes should know better. So, if we get here, the child had better be
+    // an instance of nsFrame (i.e. it should return null from GetType()).
+    // XXXdholbert Once we've fixed bug 765861, we should upgrade this to an
+    // assertion that trivially passes if bug 765861's flag has been flipped.
+    NS_WARNING_ASSERTION(
+      !aItem.Frame()->GetType(),
+      "Child should at least request space for border/padding");
+    aItem.SetCrossSize(0);
+  } else {
+    // (normal case)
+    aItem.SetCrossSize(childDesiredSize.Height() - crossAxisBorderPadding);
+  }
+
+  aItem.SetAscent(childDesiredSize.BlockStartAscent());
+}
+
+void
+FlexLine::PositionItemsInCrossAxis(nscoord aLineStartPosition,
+                                   const FlexboxAxisTracker& aAxisTracker)
+{
+  SingleLineCrossAxisPositionTracker lineCrossAxisPosnTracker(aAxisTracker);
+
+  for (FlexItem* item = mItems.getFirst(); item; item = item->getNext()) {
+    // First, stretch the item's cross size (if appropriate), and resolve any
+    // auto margins in this axis.
+    item->ResolveStretchedCrossSize(mLineCrossSize, aAxisTracker);
+    lineCrossAxisPosnTracker.ResolveAutoMarginsInCrossAxis(*this, *item);
+
+    // Compute the cross-axis position of this item
+    nscoord itemCrossBorderBoxSize =
+      item->GetCrossSize() +
+      item->GetBorderPaddingSizeInAxis(aAxisTracker.GetCrossAxis());
+    lineCrossAxisPosnTracker.EnterAlignPackingSpace(*this, *item, aAxisTracker);
+    lineCrossAxisPosnTracker.EnterMargin(item->GetMargin());
+    lineCrossAxisPosnTracker.EnterChildFrame(itemCrossBorderBoxSize);
+
+    item->SetCrossPosition(aLineStartPosition +
+                           lineCrossAxisPosnTracker.GetPosition());
+
+    // Back out to cross-axis edge of the line.
+    lineCrossAxisPosnTracker.ResetPosition();
+  }
+}
+
+void
+nsFlexContainerFrame::Reflow(nsPresContext* aPresContext,
+                             ReflowOutput& aDesiredSize,
+                             const ReflowInput& aReflowInput,
+                             nsReflowStatus& aStatus)
+{
+  MarkInReflow();
+  DO_GLOBAL_REFLOW_COUNT("nsFlexContainerFrame");
+  DISPLAY_REFLOW(aPresContext, this, aReflowInput, aDesiredSize, aStatus);
+  MOZ_LOG(gFlexContainerLog, LogLevel::Debug,
+         ("Reflow() for nsFlexContainerFrame %p\n", this));
+
+  if (IsFrameTreeTooDeep(aReflowInput, aDesiredSize, aStatus)) {
+    return;
+  }
+
+  // We (and our children) can only depend on our ancestor's bsize if we have
+  // a percent-bsize, or if we're positioned and we have "block-start" and "block-end"
+  // set and have block-size:auto.  (There are actually other cases, too -- e.g. if
+  // our parent is itself a block-dir flex container and we're flexible -- but
+  // we'll let our ancestors handle those sorts of cases.)
+  WritingMode wm = aReflowInput.GetWritingMode();
+  const nsStylePosition* stylePos = StylePosition();
+  const nsStyleCoord& bsize = stylePos->BSize(wm);
+  if (bsize.HasPercent() ||
+      (StyleDisplay()->IsAbsolutelyPositionedStyle() &&
+       eStyleUnit_Auto == bsize.GetUnit() &&
+       eStyleUnit_Auto != stylePos->mOffset.GetBStartUnit(wm) &&
+       eStyleUnit_Auto != stylePos->mOffset.GetBEndUnit(wm))) {
+    AddStateBits(NS_FRAME_CONTAINS_RELATIVE_BSIZE);
+  }
+
+  // If we've never reordered our children, then we can trust that they're
+  // already in DOM-order, and we only need to consider their "order" property
+  // when checking them for sortedness & sorting them.
+  //
+  // After we actually sort them, though, we can't trust that they're in DOM
+  // order anymore.  So, from that point on, our sort & sorted-order-checking
+  // operations need to use a fancier LEQ function that also takes DOM order
+  // into account, so that we can honor the spec's requirement that frames w/
+  // equal "order" values are laid out in DOM order.
+
+  if (!HasAnyStateBits(NS_STATE_FLEX_CHILDREN_REORDERED)) {
+    if (SortChildrenIfNeeded<IsOrderLEQ>()) {
+      AddStateBits(NS_STATE_FLEX_CHILDREN_REORDERED);
+    }
+  } else {
+    SortChildrenIfNeeded<IsOrderLEQWithDOMFallback>();
+  }
+
+  RenumberList();
+
+  const FlexboxAxisTracker axisTracker(this, aReflowInput.GetWritingMode());
+
+  // If we're being fragmented into a constrained BSize, then subtract off
+  // borderpadding BStart from that constrained BSize, to get the available
+  // BSize for our content box. (No need to subtract the borderpadding BStart
+  // if we're already skipping it via GetLogicalSkipSides, though.)
+  nscoord availableBSizeForContent = aReflowInput.AvailableBSize();
+  if (availableBSizeForContent != NS_UNCONSTRAINEDSIZE &&
+      !(GetLogicalSkipSides(&aReflowInput).BStart())) {
+    availableBSizeForContent -=
+      aReflowInput.ComputedLogicalBorderPadding().BStart(wm);
+    // (Don't let that push availableBSizeForContent below zero, though):
+    availableBSizeForContent = std::max(availableBSizeForContent, 0);
+  }
+
+  nscoord contentBoxMainSize = GetMainSizeFromReflowInput(aReflowInput,
+                                                          axisTracker);
+
+  AutoTArray<StrutInfo, 1> struts;
+  DoFlexLayout(aPresContext, aDesiredSize, aReflowInput, aStatus,
+               contentBoxMainSize, availableBSizeForContent,
+               struts, axisTracker);
+
+  if (!struts.IsEmpty()) {
+    // We're restarting flex layout, with new knowledge of collapsed items.
+    DoFlexLayout(aPresContext, aDesiredSize, aReflowInput, aStatus,
+                 contentBoxMainSize, availableBSizeForContent,
+                 struts, axisTracker);
+  }
+}
+
+// RAII class to clean up a list of FlexLines.
+// Specifically, this removes each line from the list, deletes all the
+// FlexItems in its list, and deletes the FlexLine.
+class MOZ_RAII AutoFlexLineListClearer
+{
+public:
+  explicit AutoFlexLineListClearer(LinkedList<FlexLine>& aLines
+                                   MOZ_GUARD_OBJECT_NOTIFIER_PARAM)
+  : mLines(aLines)
+  {
+    MOZ_GUARD_OBJECT_NOTIFIER_INIT;
+  }
+
+  ~AutoFlexLineListClearer()
+  {
+    while (FlexLine* line = mLines.popFirst()) {
+      while (FlexItem* item = line->mItems.popFirst()) {
+        delete item;
+      }
+      delete line;
+    }
+  }
+
+private:
+  LinkedList<FlexLine>& mLines;
+  MOZ_DECL_USE_GUARD_OBJECT_NOTIFIER
+};
+
+// Class to let us temporarily provide an override value for the the main-size
+// CSS property ('width' or 'height') on a flex item, for use in
+// nsFrame::ComputeSizeWithIntrinsicDimensions.
+// (We could use this overridden size more broadly, too, but it's probably
+// better to avoid property-table accesses.  So, where possible, we communicate
+// the resolved main-size to the child via modifying its reflow state directly,
+// instead of using this class.)
+class MOZ_RAII AutoFlexItemMainSizeOverride final
+{
+public:
+  explicit AutoFlexItemMainSizeOverride(FlexItem& aItem
+                                        MOZ_GUARD_OBJECT_NOTIFIER_PARAM)
+    : mItemProps(aItem.Frame()->Properties())
+  {
+    MOZ_GUARD_OBJECT_NOTIFIER_INIT;
+
+    MOZ_ASSERT(!mItemProps.Has(nsIFrame::FlexItemMainSizeOverride()),
+               "FlexItemMainSizeOverride prop shouldn't be set already; "
+               "it should only be set temporarily (& not recursively)");
+    NS_ASSERTION(aItem.HasIntrinsicRatio(),
+                 "This should only be needed for items with an aspect ratio");
+
+    mItemProps.Set(nsIFrame::FlexItemMainSizeOverride(), aItem.GetMainSize());
+  }
+
+  ~AutoFlexItemMainSizeOverride() {
+    mItemProps.Remove(nsIFrame::FlexItemMainSizeOverride());
+  }
+
+private:
+  const FrameProperties mItemProps;
+  MOZ_DECL_USE_GUARD_OBJECT_NOTIFIER
+};
+
+void
+nsFlexContainerFrame::CalculatePackingSpace(uint32_t aNumThingsToPack,
+                                            uint8_t aAlignVal,
+                                            nscoord* aFirstSubjectOffset,
+                                            uint32_t* aNumPackingSpacesRemaining,
+                                            nscoord* aPackingSpaceRemaining)
+{
+  MOZ_ASSERT(NS_STYLE_ALIGN_SPACE_BETWEEN == NS_STYLE_JUSTIFY_SPACE_BETWEEN &&
+             NS_STYLE_ALIGN_SPACE_AROUND == NS_STYLE_JUSTIFY_SPACE_AROUND &&
+             NS_STYLE_ALIGN_SPACE_EVENLY == NS_STYLE_JUSTIFY_SPACE_EVENLY,
+             "CalculatePackingSpace assumes that NS_STYLE_ALIGN_SPACE and "
+             "NS_STYLE_JUSTIFY_SPACE constants are interchangeable");
+
+  MOZ_ASSERT(aAlignVal == NS_STYLE_ALIGN_SPACE_BETWEEN ||
+             aAlignVal == NS_STYLE_ALIGN_SPACE_AROUND ||
+             aAlignVal == NS_STYLE_ALIGN_SPACE_EVENLY,
+             "Unexpected alignment value");
+
+  MOZ_ASSERT(*aPackingSpaceRemaining >= 0,
+             "Should not be called with negative packing space");
+
+  MOZ_ASSERT(aNumThingsToPack >= 1,
+             "Should not be called with less than 1 thing to pack");
+
+  // Packing spaces between items:
+  *aNumPackingSpacesRemaining = aNumThingsToPack - 1;
+
+  if (aAlignVal == NS_STYLE_ALIGN_SPACE_BETWEEN) {
+    // No need to reserve space at beginning/end, so we're done.
+    return;
+  }
+
+  // We need to add 1 or 2 packing spaces, split between beginning/end, for
+  // space-around / space-evenly:
+  size_t numPackingSpacesForEdges =
+    aAlignVal == NS_STYLE_JUSTIFY_SPACE_AROUND ? 1 : 2;
+
+  // How big will each "full" packing space be:
+  nscoord packingSpaceSize = *aPackingSpaceRemaining /
+    (*aNumPackingSpacesRemaining + numPackingSpacesForEdges);
+  // How much packing-space are we allocating to the edges:
+  nscoord totalEdgePackingSpace = numPackingSpacesForEdges * packingSpaceSize;
+
+  // Use half of that edge packing space right now:
+  *aFirstSubjectOffset += totalEdgePackingSpace / 2;
+  // ...but we need to subtract all of it right away, so that we won't
+  // hand out any of it to intermediate packing spaces.
+  *aPackingSpaceRemaining -= totalEdgePackingSpace;
+}
+
+void
+nsFlexContainerFrame::DoFlexLayout(nsPresContext*           aPresContext,
+                                   ReflowOutput&     aDesiredSize,
+                                   const ReflowInput& aReflowInput,
+                                   nsReflowStatus&          aStatus,
+                                   nscoord aContentBoxMainSize,
+                                   nscoord aAvailableBSizeForContent,
+                                   nsTArray<StrutInfo>& aStruts,
+                                   const FlexboxAxisTracker& aAxisTracker)
+{
+  aStatus = NS_FRAME_COMPLETE;
+
+  LinkedList<FlexLine> lines;
+  nsTArray<nsIFrame*> placeholderKids;
+  AutoFlexLineListClearer cleanupLines(lines);
+
+  GenerateFlexLines(aPresContext, aReflowInput,
+                    aContentBoxMainSize,
+                    aAvailableBSizeForContent,
+                    aStruts, aAxisTracker,
+                    placeholderKids, lines);
+
+  if (lines.getFirst()->IsEmpty() &&
+      !lines.getFirst()->getNext()) {
+    // We have no flex items, our parent should synthesize a baseline if needed.
+    AddStateBits(NS_STATE_FLEX_SYNTHESIZE_BASELINE);
+  } else {
+    RemoveStateBits(NS_STATE_FLEX_SYNTHESIZE_BASELINE);
+  }
+
+  aContentBoxMainSize =
+    ResolveFlexContainerMainSize(aReflowInput, aAxisTracker,
+                                 aContentBoxMainSize, aAvailableBSizeForContent,
+                                 lines.getFirst(), aStatus);
+
+  for (FlexLine* line = lines.getFirst(); line; line = line->getNext()) {
+    line->ResolveFlexibleLengths(aContentBoxMainSize);
+  }
+
+  // Cross Size Determination - Flexbox spec section 9.4
+  // ===================================================
+  // Calculate the hypothetical cross size of each item:
+  nscoord sumLineCrossSizes = 0;
+  for (FlexLine* line = lines.getFirst(); line; line = line->getNext()) {
+    for (FlexItem* item = line->GetFirstItem(); item; item = item->getNext()) {
+      // The item may already have the correct cross-size; only recalculate
+      // if the item's main size resolution (flexing) could have influenced it:
+      if (item->CanMainSizeInfluenceCrossSize(aAxisTracker)) {
+        Maybe<AutoFlexItemMainSizeOverride> sizeOverride;
+        if (item->HasIntrinsicRatio()) {
+          // For flex items with an aspect ratio, we have to impose an override
+          // for the main-size property *before* we even instantiate the reflow
+          // state, in order for aspect ratio calculations to produce the right
+          // cross size in the reflow state. (For other flex items, it's OK
+          // (and cheaper) to impose our main size *after* the reflow state has
+          // been constructed, since the main size shouldn't influence anything
+          // about cross-size measurement until we actually reflow the child.)
+          sizeOverride.emplace(*item);
+        }
+
+        WritingMode wm = item->Frame()->GetWritingMode();
+        LogicalSize availSize = aReflowInput.ComputedSize(wm);
+        availSize.BSize(wm) = NS_UNCONSTRAINEDSIZE;
+        ReflowInput childReflowInput(aPresContext, aReflowInput,
+                                           item->Frame(), availSize);
+        if (!sizeOverride) {
+          // Directly override the computed main-size, by tweaking reflow state:
+          if (aAxisTracker.IsMainAxisHorizontal()) {
+            childReflowInput.SetComputedWidth(item->GetMainSize());
+          } else {
+            childReflowInput.SetComputedHeight(item->GetMainSize());
+          }
+        }
+
+        SizeItemInCrossAxis(aPresContext, aAxisTracker,
+                            childReflowInput, *item);
+      }
+    }
+    // Now that we've finished with this line's items, size the line itself:
+    line->ComputeCrossSizeAndBaseline(aAxisTracker);
+    sumLineCrossSizes += line->GetLineCrossSize();
+  }
+
+  bool isCrossSizeDefinite;
+  const nscoord contentBoxCrossSize =
+    ComputeCrossSize(aReflowInput, aAxisTracker, sumLineCrossSizes,
+                     aAvailableBSizeForContent, &isCrossSizeDefinite, aStatus);
+
+  // Set up state for cross-axis alignment, at a high level (outside the
+  // scope of a particular flex line)
+  CrossAxisPositionTracker
+    crossAxisPosnTracker(lines.getFirst(),
+                         aReflowInput, contentBoxCrossSize,
+                         isCrossSizeDefinite, aAxisTracker);
+
+  // Now that we know the cross size of each line (including
+  // "align-content:stretch" adjustments, from the CrossAxisPositionTracker
+  // constructor), we can create struts for any flex items with
+  // "visibility: collapse" (and restart flex layout).
+  if (aStruts.IsEmpty()) { // (Don't make struts if we already did)
+    BuildStrutInfoFromCollapsedItems(lines.getFirst(), aStruts);
+    if (!aStruts.IsEmpty()) {
+      // Restart flex layout, using our struts.
+      return;
+    }
+  }
+
+  // If the container should derive its baseline from the first FlexLine,
+  // do that here (while crossAxisPosnTracker is conveniently pointing
+  // at the cross-start edge of that line, which the line's baseline offset is
+  // measured from):
+  nscoord flexContainerAscent;
+  if (!aAxisTracker.AreAxesInternallyReversed()) {
+    nscoord firstLineBaselineOffset = lines.getFirst()->GetFirstBaselineOffset();
+    if (firstLineBaselineOffset == nscoord_MIN) {
+      // No baseline-aligned items in line. Use sentinel value to prompt us to
+      // get baseline from the first FlexItem after we've reflowed it.
+      flexContainerAscent = nscoord_MIN;
+    } else  {
+      flexContainerAscent =
+        ComputePhysicalAscentFromFlexRelativeAscent(
+          crossAxisPosnTracker.GetPosition() + firstLineBaselineOffset,
+          contentBoxCrossSize, aReflowInput, aAxisTracker);
+    }
+  }
+
+  const auto justifyContent = IsLegacyBox(aReflowInput.mFrame) ?
+    ConvertLegacyStyleToJustifyContent(StyleXUL()) :
+    aReflowInput.mStylePosition->mJustifyContent;
+
+  for (FlexLine* line = lines.getFirst(); line; line = line->getNext()) {
+    // Main-Axis Alignment - Flexbox spec section 9.5
+    // ==============================================
+    line->PositionItemsInMainAxis(justifyContent,
+                                  aContentBoxMainSize,
+                                  aAxisTracker);
+
+    // Cross-Axis Alignment - Flexbox spec section 9.6
+    // ===============================================
+    line->PositionItemsInCrossAxis(crossAxisPosnTracker.GetPosition(),
+                                   aAxisTracker);
+    crossAxisPosnTracker.TraverseLine(*line);
+    crossAxisPosnTracker.TraversePackingSpace();
+  }
+
+  // If the container should derive its baseline from the last FlexLine,
+  // do that here (while crossAxisPosnTracker is conveniently pointing
+  // at the cross-end edge of that line, which the line's baseline offset is
+  // measured from):
+  if (aAxisTracker.AreAxesInternallyReversed()) {
+    nscoord lastLineBaselineOffset = lines.getLast()->GetFirstBaselineOffset();
+    if (lastLineBaselineOffset == nscoord_MIN) {
+      // No baseline-aligned items in line. Use sentinel value to prompt us to
+      // get baseline from the last FlexItem after we've reflowed it.
+      flexContainerAscent = nscoord_MIN;
+    } else {
+      flexContainerAscent =
+        ComputePhysicalAscentFromFlexRelativeAscent(
+          crossAxisPosnTracker.GetPosition() - lastLineBaselineOffset,
+          contentBoxCrossSize, aReflowInput, aAxisTracker);
+    }
+  }
+
+  // Before giving each child a final reflow, calculate the origin of the
+  // flex container's content box (with respect to its border-box), so that
+  // we can compute our flex item's final positions.
+  WritingMode flexWM = aReflowInput.GetWritingMode();
+  LogicalMargin containerBP = aReflowInput.ComputedLogicalBorderPadding();
+
+  // Unconditionally skip block-end border & padding for now, regardless of
+  // writing-mode/GetLogicalSkipSides.  We add it lower down, after we've
+  // established baseline and decided whether bottom border-padding fits (if
+  // we're fragmented).
+  const nscoord blockEndContainerBP = containerBP.BEnd(flexWM);
+  const LogicalSides skipSides =
+    GetLogicalSkipSides(&aReflowInput) | LogicalSides(eLogicalSideBitsBEnd);
+  containerBP.ApplySkipSides(skipSides);
+
+  const LogicalPoint containerContentBoxOrigin(flexWM,
+                                               containerBP.IStart(flexWM),
+                                               containerBP.BStart(flexWM));
+
+  // Determine flex container's border-box size (used in positioning children):
+  LogicalSize logSize =
+    aAxisTracker.LogicalSizeFromFlexRelativeSizes(aContentBoxMainSize,
+                                                  contentBoxCrossSize);
+  logSize += aReflowInput.ComputedLogicalBorderPadding().Size(flexWM);
+  nsSize containerSize = logSize.GetPhysicalSize(flexWM);
+
+  // If the flex container has no baseline-aligned items, it will use this item
+  // (the first item, discounting any under-the-hood reversing that we've done)
+  // to determine its baseline:
+  const FlexItem* const firstItem =
+    aAxisTracker.AreAxesInternallyReversed()
+    ? lines.getLast()->GetLastItem()
+    : lines.getFirst()->GetFirstItem();
+
+  // FINAL REFLOW: Give each child frame another chance to reflow, now that
+  // we know its final size and position.
+  for (const FlexLine* line = lines.getFirst(); line; line = line->getNext()) {
+    for (const FlexItem* item = line->GetFirstItem(); item;
+         item = item->getNext()) {
+      LogicalPoint framePos = aAxisTracker.LogicalPointFromFlexRelativePoint(
+                               item->GetMainPosition(),
+                               item->GetCrossPosition(),
+                               aContentBoxMainSize,
+                               contentBoxCrossSize);
+      // Adjust framePos to be relative to the container's border-box
+      // (i.e. its frame rect), instead of the container's content-box:
+      framePos += containerContentBoxOrigin;
+
+      // (Intentionally snapshotting this before ApplyRelativePositioning, to
+      // maybe use for setting the flex container's baseline.)
+      const nscoord itemNormalBPos = framePos.B(flexWM);
+
+      // Check if we actually need to reflow the item -- if we already reflowed
+      // it with the right size, we can just reposition it as-needed.
+      bool itemNeedsReflow = true; // (Start out assuming the worst.)
+      if (item->HadMeasuringReflow()) {
+        LogicalSize finalFlexItemCBSize =
+          aAxisTracker.LogicalSizeFromFlexRelativeSizes(item->GetMainSize(),
+                                                        item->GetCrossSize());
+        // We've already reflowed the child once. Was the size we gave it in
+        // that reflow the same as its final (post-flexing/stretching) size?
+        if (finalFlexItemCBSize ==
+            LogicalSize(flexWM,
+                        item->Frame()->GetContentRectRelativeToSelf().Size())) {
+          // Even if our size hasn't changed, some of our descendants might
+          // care that our bsize is now considered "definite" (whereas it
+          // wasn't in our previous "measuring" reflow), if they have a
+          // relative bsize.
+          if (!(item->Frame()->GetStateBits() &
+                NS_FRAME_CONTAINS_RELATIVE_BSIZE)) {
+            // Item has the correct size (and its children don't care that
+            // it's now "definite"). Let's just make sure it's at the right
+            // position.
+            itemNeedsReflow = false;
+            MoveFlexItemToFinalPosition(aReflowInput, *item, framePos,
+                                        containerSize);
+          }
+        }
+      }
+      if (itemNeedsReflow) {
+        ReflowFlexItem(aPresContext, aAxisTracker, aReflowInput,
+                       *item, framePos, containerSize);
+      }
+
+      // If this is our first item and we haven't established a baseline for
+      // the container yet (i.e. if we don't have 'align-self: baseline' on any
+      // children), then use this child's first baseline as the container's
+      // baseline.
+      if (item == firstItem &&
+          flexContainerAscent == nscoord_MIN) {
+        flexContainerAscent = itemNormalBPos + item->ResolvedAscent(true);
+      }
+    }
+  }
+
+  if (!placeholderKids.IsEmpty()) {
+    ReflowPlaceholders(aPresContext, aReflowInput,
+                       placeholderKids, containerContentBoxOrigin,
+                       containerSize);
+  }
+
+  // Compute flex container's desired size (in its own writing-mode),
+  // starting w/ content-box size & growing from there:
+  LogicalSize desiredSizeInFlexWM =
+    aAxisTracker.LogicalSizeFromFlexRelativeSizes(aContentBoxMainSize,
+                                                  contentBoxCrossSize);
+  // Add border/padding (w/ skipSides already applied):
+  desiredSizeInFlexWM.ISize(flexWM) += containerBP.IStartEnd(flexWM);
+  desiredSizeInFlexWM.BSize(flexWM) += containerBP.BStartEnd(flexWM);
+
+  if (flexContainerAscent == nscoord_MIN) {
+    // Still don't have our baseline set -- this happens if we have no
+    // children (or if our children are huge enough that they have nscoord_MIN
+    // as their baseline... in which case, we'll use the wrong baseline, but no
+    // big deal)
+    NS_WARNING_ASSERTION(
+      lines.getFirst()->IsEmpty(),
+      "Have flex items but didn't get an ascent - that's odd (or there are "
+      "just gigantic sizes involved)");
+    // Per spec, synthesize baseline from the flex container's content box
+    // (i.e. use block-end side of content-box)
+    // XXXdholbert This only makes sense if parent's writing mode is
+    // horizontal (& even then, really we should be using the BSize in terms
+    // of the parent's writing mode, not ours). Clean up in bug 1155322.
+    flexContainerAscent = desiredSizeInFlexWM.BSize(flexWM);
+  }
+
+  if (HasAnyStateBits(NS_STATE_FLEX_SYNTHESIZE_BASELINE)) {
+    // This will force our parent to call GetLogicalBaseline, which will
+    // synthesize a margin-box baseline.
+    aDesiredSize.SetBlockStartAscent(ReflowOutput::ASK_FOR_BASELINE);
+  } else {
+    // XXXdholbert flexContainerAscent needs to be in terms of
+    // our parent's writing-mode here. See bug 1155322.
+    aDesiredSize.SetBlockStartAscent(flexContainerAscent);
+  }
+
+  // Now: If we're complete, add bottom border/padding to desired height (which
+  // we skipped via skipSides) -- unless that pushes us over available height,
+  // in which case we become incomplete (unless we already weren't asking for
+  // any height, in which case we stay complete to avoid looping forever).
+  // NOTE: If we're auto-height, we allow our bottom border/padding to push us
+  // over the available height without requesting a continuation, for
+  // consistency with the behavior of "display:block" elements.
+  if (NS_FRAME_IS_COMPLETE(aStatus)) {
+    nscoord desiredBSizeWithBEndBP =
+      desiredSizeInFlexWM.BSize(flexWM) + blockEndContainerBP;
+
+    if (aReflowInput.AvailableBSize() == NS_UNCONSTRAINEDSIZE ||
+        desiredSizeInFlexWM.BSize(flexWM) == 0 ||
+        desiredBSizeWithBEndBP <= aReflowInput.AvailableBSize() ||
+        aReflowInput.ComputedBSize() == NS_INTRINSICSIZE) {
+      // Update desired height to include block-end border/padding
+      desiredSizeInFlexWM.BSize(flexWM) = desiredBSizeWithBEndBP;
+    } else {
+      // We couldn't fit bottom border/padding, so we'll need a continuation.
+      NS_FRAME_SET_INCOMPLETE(aStatus);
+    }
+  }
+
+  // Calculate the container baselines so that our parent can baseline-align us.
+  mBaselineFromLastReflow = flexContainerAscent;
+  mLastBaselineFromLastReflow = lines.getLast()->GetLastBaselineOffset();
+  if (mLastBaselineFromLastReflow == nscoord_MIN) {
+    // XXX we fall back to a mirrored first baseline here for now, but this
+    // should probably use the last baseline of the last item or something.
+    mLastBaselineFromLastReflow =
+      desiredSizeInFlexWM.BSize(flexWM) - flexContainerAscent;
+  }
+
+  // Convert flex container's final desired size to parent's WM, for outparam.
+  aDesiredSize.SetSize(flexWM, desiredSizeInFlexWM);
+
+  // Overflow area = union(my overflow area, kids' overflow areas)
+  aDesiredSize.SetOverflowAreasToDesiredBounds();
+  for (nsIFrame* childFrame : mFrames) {
+    ConsiderChildOverflow(aDesiredSize.mOverflowAreas, childFrame);
+  }
+
+  FinishReflowWithAbsoluteFrames(aPresContext, aDesiredSize,
+                                 aReflowInput, aStatus);
+
+  NS_FRAME_SET_TRUNCATION(aStatus, aReflowInput, aDesiredSize)
+}
+
+void
+nsFlexContainerFrame::MoveFlexItemToFinalPosition(
+  const ReflowInput& aReflowInput,
+  const FlexItem& aItem,
+  LogicalPoint& aFramePos,
+  const nsSize& aContainerSize)
+{
+  WritingMode outerWM = aReflowInput.GetWritingMode();
+
+  // If item is relpos, look up its offsets (cached from prev reflow)
+  LogicalMargin logicalOffsets(outerWM);
+  if (NS_STYLE_POSITION_RELATIVE == aItem.Frame()->StyleDisplay()->mPosition) {
+    FrameProperties props = aItem.Frame()->Properties();
+    nsMargin* cachedOffsets = props.Get(nsIFrame::ComputedOffsetProperty());
+    MOZ_ASSERT(cachedOffsets,
+               "relpos previously-reflowed frame should've cached its offsets");
+    logicalOffsets = LogicalMargin(outerWM, *cachedOffsets);
+  }
+  ReflowInput::ApplyRelativePositioning(aItem.Frame(), outerWM,
+                                              logicalOffsets, &aFramePos,
+                                              aContainerSize);
+  aItem.Frame()->SetPosition(outerWM, aFramePos, aContainerSize);
+  PositionFrameView(aItem.Frame());
+  PositionChildViews(aItem.Frame());
+}
+
+void
+nsFlexContainerFrame::ReflowFlexItem(nsPresContext* aPresContext,
+                                     const FlexboxAxisTracker& aAxisTracker,
+                                     const ReflowInput& aReflowInput,
+                                     const FlexItem& aItem,
+                                     LogicalPoint& aFramePos,
+                                     const nsSize& aContainerSize)
+{
+  WritingMode outerWM = aReflowInput.GetWritingMode();
+  WritingMode wm = aItem.Frame()->GetWritingMode();
+  LogicalSize availSize = aReflowInput.ComputedSize(wm);
+  availSize.BSize(wm) = NS_UNCONSTRAINEDSIZE;
+  ReflowInput childReflowInput(aPresContext, aReflowInput,
+                                     aItem.Frame(), availSize);
+
+  // Keep track of whether we've overriden the child's computed height
+  // and/or width, so we can set its resize flags accordingly.
+  bool didOverrideComputedWidth = false;
+  bool didOverrideComputedHeight = false;
+
+  // Override computed main-size
+  if (aAxisTracker.IsMainAxisHorizontal()) {
+    childReflowInput.SetComputedWidth(aItem.GetMainSize());
+    didOverrideComputedWidth = true;
+  } else {
+    childReflowInput.SetComputedHeight(aItem.GetMainSize());
+    didOverrideComputedHeight = true;
+  }
+
+  // Override reflow state's computed cross-size if either:
+  // - the item was stretched (in which case we're imposing a cross size)
+  // ...or...
+  // - the item it has an aspect ratio (in which case the cross-size that's
+  // currently in the reflow state is based on arithmetic involving a stale
+  // main-size value that we just stomped on above). (Note that we could handle
+  // this case using an AutoFlexItemMainSizeOverride, as we do elsewhere; but
+  // given that we *already know* the correct cross size to use here, it's
+  // cheaper to just directly set it instead of setting a frame property.)
+  if (aItem.IsStretched() ||
+      aItem.HasIntrinsicRatio()) {
+    if (aAxisTracker.IsCrossAxisHorizontal()) {
+      childReflowInput.SetComputedWidth(aItem.GetCrossSize());
+      didOverrideComputedWidth = true;
+    } else {
+      childReflowInput.SetComputedHeight(aItem.GetCrossSize());
+      didOverrideComputedHeight = true;
+    }
+  }
+  if (aItem.IsStretched() && !aAxisTracker.IsCrossAxisHorizontal()) {
+    // If this item's height is stretched, it's a relative height.
+    aItem.Frame()->AddStateBits(NS_FRAME_CONTAINS_RELATIVE_BSIZE);
+  }
+
+  // XXXdholbert Might need to actually set the correct margins in the
+  // reflow state at some point, so that they can be saved on the frame for
+  // UsedMarginProperty().  Maybe doesn't matter though...?
+
+  // If we're overriding the computed width or height, *and* we had an
+  // earlier "measuring" reflow, then this upcoming reflow needs to be
+  // treated as a resize.
+  if (aItem.HadMeasuringReflow()) {
+    if (didOverrideComputedWidth) {
+      // (This is somewhat redundant, since the reflow state already
+      // sets mHResize whenever our computed width has changed since the
+      // previous reflow. Still, it's nice for symmetry, and it may become
+      // necessary once we support orthogonal flows.)
+      childReflowInput.SetHResize(true);
+    }
+    if (didOverrideComputedHeight) {
+      childReflowInput.SetVResize(true);
+    }
+  }
+  // NOTE: Be very careful about doing anything else with childReflowInput
+  // after this point, because some of its methods (e.g. SetComputedWidth)
+  // internally call InitResizeFlags and stomp on mVResize & mHResize.
+
+  ReflowOutput childDesiredSize(childReflowInput);
+  nsReflowStatus childReflowStatus;
+  ReflowChild(aItem.Frame(), aPresContext,
+              childDesiredSize, childReflowInput,
+              outerWM, aFramePos, aContainerSize,
+              0, childReflowStatus);
+
+  // XXXdholbert Once we do pagination / splitting, we'll need to actually
+  // handle incomplete childReflowStatuses. But for now, we give our kids
+  // unconstrained available height, which means they should always
+  // complete.
+  MOZ_ASSERT(NS_FRAME_IS_COMPLETE(childReflowStatus),
+             "We gave flex item unconstrained available height, so it "
+             "should be complete");
+
+  LogicalMargin offsets =
+    childReflowInput.ComputedLogicalOffsets().ConvertTo(outerWM, wm);
+  ReflowInput::ApplyRelativePositioning(aItem.Frame(), outerWM,
+                                              offsets, &aFramePos,
+                                              aContainerSize);
+
+  FinishReflowChild(aItem.Frame(), aPresContext,
+                    childDesiredSize, &childReflowInput,
+                    outerWM, aFramePos, aContainerSize, 0);
+
+  aItem.SetAscent(childDesiredSize.BlockStartAscent());
+}
+
+void
+nsFlexContainerFrame::ReflowPlaceholders(nsPresContext* aPresContext,
+                                         const ReflowInput& aReflowInput,
+                                         nsTArray<nsIFrame*>& aPlaceholders,
+                                         const LogicalPoint& aContentBoxOrigin,
+                                         const nsSize& aContainerSize)
+{
+  WritingMode outerWM = aReflowInput.GetWritingMode();
+
+  // As noted in this method's documentation, we'll reflow every entry in
+  // |aPlaceholders| at the container's content-box origin.
+  for (nsIFrame* placeholder : aPlaceholders) {
+    MOZ_ASSERT(placeholder->GetType() == nsGkAtoms::placeholderFrame,
+               "placeholders array should only contain placeholder frames");
+    WritingMode wm = placeholder->GetWritingMode();
+    LogicalSize availSize = aReflowInput.ComputedSize(wm);
+    ReflowInput childReflowInput(aPresContext, aReflowInput,
+                                 placeholder, availSize);
+    ReflowOutput childDesiredSize(childReflowInput);
+    nsReflowStatus childReflowStatus;
+    ReflowChild(placeholder, aPresContext,
+                childDesiredSize, childReflowInput,
+                outerWM, aContentBoxOrigin, aContainerSize, 0,
+                childReflowStatus);
+
+    FinishReflowChild(placeholder, aPresContext,
+                      childDesiredSize, &childReflowInput,
+                      outerWM, aContentBoxOrigin, aContainerSize, 0);
+
+    // Mark the placeholder frame to indicate that it's not actually at the
+    // element's static position, because we need to apply CSS Alignment after
+    // we determine the OOF's size:
+    placeholder->AddStateBits(PLACEHOLDER_STATICPOS_NEEDS_CSSALIGN);
+  }
+}
+
+/* virtual */ nscoord
+nsFlexContainerFrame::GetMinISize(nsRenderingContext* aRenderingContext)
+{
+  nscoord minISize = 0;
+  DISPLAY_MIN_WIDTH(this, minISize);
+
+  RenumberList();
+
+  const nsStylePosition* stylePos = StylePosition();
+  const FlexboxAxisTracker axisTracker(this, GetWritingMode());
+
+  for (nsIFrame* childFrame : mFrames) {
+    nscoord childMinISize =
+      nsLayoutUtils::IntrinsicForContainer(aRenderingContext, childFrame,
+                                           nsLayoutUtils::MIN_ISIZE);
+    // For a horizontal single-line flex container, the intrinsic min
+    // isize is the sum of its items' min isizes.
+    // For a column-oriented flex container, or for a multi-line row-
+    // oriented flex container, the intrinsic min isize is the max of
+    // its items' min isizes.
+    if (axisTracker.IsRowOriented() &&
+        NS_STYLE_FLEX_WRAP_NOWRAP == stylePos->mFlexWrap) {
+      minISize += childMinISize;
+    } else {
+      minISize = std::max(minISize, childMinISize);
+    }
+  }
+  return minISize;
+}
+
+/* virtual */ nscoord
+nsFlexContainerFrame::GetPrefISize(nsRenderingContext* aRenderingContext)
+{
+  nscoord prefISize = 0;
+  DISPLAY_PREF_WIDTH(this, prefISize);
+
+  RenumberList();
+
+  // XXXdholbert Optimization: We could cache our intrinsic widths like
+  // nsBlockFrame does (and return it early from this function if it's set).
+  // Whenever anything happens that might change it, set it to
+  // NS_INTRINSIC_WIDTH_UNKNOWN (like nsBlockFrame::MarkIntrinsicISizesDirty
+  // does)
+  const FlexboxAxisTracker axisTracker(this, GetWritingMode());
+
+  for (nsIFrame* childFrame : mFrames) {
+    nscoord childPrefISize =
+      nsLayoutUtils::IntrinsicForContainer(aRenderingContext, childFrame,
+                                           nsLayoutUtils::PREF_ISIZE);
+    if (axisTracker.IsRowOriented()) {
+      prefISize += childPrefISize;
+    } else {
+      prefISize = std::max(prefISize, childPrefISize);
+    }
+  }
+  return prefISize;
+}