5 files changed, 41 insertions, 4 deletions

diff --git a/dom/base/nsIContent.h b/dom/base/nsIContent.h
index ff0c57e8c..edb0d69f3 100644
--- a/dom/base/nsIContent.h
+++ b/dom/base/nsIContent.h
@@ -950,6 +950,14 @@ public:
     return false;
   }
 
+  // Returns true if this element is native-anonymous scrollbar content.
+  bool IsNativeScrollbarContent() const {
+    return IsNativeAnonymous() &&
+           IsAnyOfXULElements(nsGkAtoms::scrollbar,
+                              nsGkAtoms::resizer,
+                              nsGkAtoms::scrollcorner);
+  }
+
   // Overloaded from nsINode
   virtual already_AddRefed<nsIURI> GetBaseURI(bool aTryUseXHRDocBaseURI = false) const override;
 
diff --git a/layout/base/nsCSSFrameConstructor.cpp b/layout/base/nsCSSFrameConstructor.cpp
index a2dec55e1..e88b2d62c 100644
--- a/layout/base/nsCSSFrameConstructor.cpp
+++ b/layout/base/nsCSSFrameConstructor.cpp
@@ -10730,6 +10730,12 @@ nsCSSFrameConstructor::AddFCItemsForAnonymousContent(
     // first element that is not itself NAC (distinct from whether it happens
     // to be in a NAC subtree).
     //
+    // The one exception to all of this is scrollbar content, which we parent
+    // directly to the scrollframe. This is because the special-snowflake
+    // construction of scroll frames doesn't result in the placeholder frame
+    // being constructed until later, which means that GetInFlowParent() doesn't
+    // work right in the case of out-of-flow scrollframes.
+    //
     // To implement all this, we need to pass the correct parent style context
     // here because SetPrimaryFrame() may not have been called on the content
     // yet and thus ResolveStyleContext can't find it otherwise.
@@ -10739,9 +10745,12 @@ nsCSSFrameConstructor::AddFCItemsForAnonymousContent(
     // to worry about anonymous boxes, which CorrectStyleParentFrame handles
     // for us.
     nsIFrame* inheritFrame = aFrame;
-    while (inheritFrame->GetContent()->IsNativeAnonymous()) {
-      inheritFrame = inheritFrame->GetParent();
+    if (!content->IsNativeScrollbarContent()) {
+      while (inheritFrame->GetContent()->IsNativeAnonymous()) {
+        inheritFrame = inheritFrame->GetInFlowParent();
+      }
     }
+
     if (inheritFrame->GetType() == nsGkAtoms::canvasFrame) {
       // CorrectStyleParentFrame returns nullptr if the prospective parent is
       // the canvas frame, so avoid calling it in that situation.
diff --git a/layout/generic/nsFrame.cpp b/layout/generic/nsFrame.cpp
index 2b81ce83b..71f6172bd 100644
--- a/layout/generic/nsFrame.cpp
+++ b/layout/generic/nsFrame.cpp
@@ -8964,10 +8964,10 @@ GetCorrectedParent(const nsIFrame* aFrame)
   nsIContent* content = aFrame->GetContent();
   Element* element =
     content && content->IsElement() ? content->AsElement() : nullptr;
-  if (element && element->IsNativeAnonymous() &&
+  if (element && element->IsNativeAnonymous() && !element->IsNativeScrollbarContent() &&
       element->GetPseudoElementType() == aFrame->StyleContext()->GetPseudoType()) {
     while (parent->GetContent() && parent->GetContent()->IsNativeAnonymous()) {
-      parent = parent->GetParent();
+      parent = parent->GetInFlowParent();
     }
   }
 
diff --git a/layout/generic/nsIFrame.h b/layout/generic/nsIFrame.h
index 93eb95099..3137aaee2 100644
--- a/layout/generic/nsIFrame.h
+++ b/layout/generic/nsIFrame.h
@@ -721,6 +721,13 @@ public:
    * Accessor functions for geometric parent.
    */
   nsContainerFrame* GetParent() const { return mParent; }
+
+  /**
+   * Gets the parent of a frame, using the parent of the placeholder for
+   * out-of-flow frames.
+   */
+  inline nsContainerFrame* GetInFlowParent();
+
   /**
    * Set this frame's parent to aParent.
    * If the frame may have moved into or out of a scrollframe's
diff --git a/layout/generic/nsIFrameInlines.h b/layout/generic/nsIFrameInlines.h
index eb9a7202a..3068c9f79 100644
--- a/layout/generic/nsIFrameInlines.h
+++ b/layout/generic/nsIFrameInlines.h
@@ -8,8 +8,10 @@
 #define nsIFrameInlines_h___
 
 #include "nsContainerFrame.h"
+#include "nsPlaceholderFrame.h"
 #include "nsStyleStructInlines.h"
 #include "nsCSSAnonBoxes.h"
+#include "nsFrameManager.h"
 
 bool
 nsIFrame::IsFlexItem() const
@@ -160,4 +162,15 @@ nsIFrame::BaselineBOffset(mozilla::WritingMode aWM,
   return SynthesizeBaselineBOffsetFromBorderBox(aWM, aBaselineGroup);
 }
 
+nsContainerFrame*
+nsIFrame::GetInFlowParent()
+{
+  if (GetStateBits() & NS_FRAME_OUT_OF_FLOW) {
+    nsFrameManager* fm = PresContext()->FrameManager();
+    return fm->GetPlaceholderFrameFor(FirstContinuation())->GetParent();
+  }
+
+  return GetParent();
+}
+
 #endif