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diff --git a/layout/mathml/nsMathMLmfracFrame.h b/layout/mathml/nsMathMLmfracFrame.h
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+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef nsMathMLmfracFrame_h___
+#define nsMathMLmfracFrame_h___
+
+#include "mozilla/Attributes.h"
+#include "nsMathMLContainerFrame.h"
+
+//
+// <mfrac> -- form a fraction from two subexpressions 
+//
+
+/*
+The MathML REC describes:
+
+The <mfrac> element is used for fractions. It can also be used to mark up 
+fraction-like objects such as binomial coefficients and Legendre symbols. 
+The syntax for <mfrac> is:
+  <mfrac> numerator denominator </mfrac>
+
+Attributes of <mfrac>:
+     Name                      values                     default 
+  linethickness  number [ v-unit ] | thin | medium | thick  1 
+
+E.g., 
+linethickness=2 actually means that linethickness=2*DEFAULT_THICKNESS
+(DEFAULT_THICKNESS is not specified by MathML, see below.)
+
+The linethickness attribute indicates the thickness of the horizontal
+"fraction bar", or "rule", typically used to render fractions. A fraction
+with linethickness="0" renders without the bar, and might be used within
+binomial coefficients. A linethickness greater than one might be used with
+nested fractions. 
+
+In general, the value of linethickness can be a number, as a multiplier
+of the default thickness of the fraction bar (the default thickness is
+not specified by MathML), or a number with a unit of vertical length (see
+Section 2.3.3), or one of the keywords medium (same as 1), thin (thinner
+than 1, otherwise up to the renderer), or thick (thicker than 1, otherwise
+up to the renderer). 
+
+The <mfrac> element sets displaystyle to "false", or if it was already
+false increments scriptlevel by 1, within numerator and denominator.
+These attributes are inherited by every element from its rendering 
+environment, but can be set explicitly only on the <mstyle> 
+element. 
+*/
+
+class nsMathMLmfracFrame : public nsMathMLContainerFrame {
+public:
+  NS_DECL_FRAMEARENA_HELPERS
+
+  friend nsIFrame* NS_NewMathMLmfracFrame(nsIPresShell* aPresShell, nsStyleContext* aContext);
+
+  virtual eMathMLFrameType GetMathMLFrameType() override;
+
+  virtual nsresult
+  MeasureForWidth(DrawTarget* aDrawTarget,
+                  ReflowOutput& aDesiredSize) override;
+
+  virtual nsresult
+  Place(DrawTarget*          aDrawTarget,
+        bool                 aPlaceOrigin,
+        ReflowOutput& aDesiredSize) override;
+
+  virtual void BuildDisplayList(nsDisplayListBuilder*   aBuilder,
+                                const nsRect&           aDirtyRect,
+                                const nsDisplayListSet& aLists) override;
+
+  virtual nsresult
+  AttributeChanged(int32_t  aNameSpaceID,
+                   nsIAtom* aAttribute,
+                   int32_t  aModType) override;
+
+  NS_IMETHOD
+  TransmitAutomaticData() override;
+
+  // override the base method so that we can deal with the fraction line
+  virtual nscoord
+  FixInterFrameSpacing(ReflowOutput& aDesiredSize) override;
+
+  // helper to translate the thickness attribute into a usable form
+  static nscoord 
+  CalcLineThickness(nsPresContext*  aPresContext,
+                    nsStyleContext*  aStyleContext,
+                    nsString&        aThicknessAttribute,
+                    nscoord          onePixel,
+                    nscoord          aDefaultRuleThickness,
+                    float            aFontSizeInflation);
+
+  uint8_t
+  ScriptIncrement(nsIFrame* aFrame) override;
+
+protected:
+  explicit nsMathMLmfracFrame(nsStyleContext* aContext) : nsMathMLContainerFrame(aContext) {}
+  virtual ~nsMathMLmfracFrame();
+
+  nsresult PlaceInternal(DrawTarget*          aDrawTarget,
+                         bool                 aPlaceOrigin,
+                         ReflowOutput& aDesiredSize,
+                         bool                 aWidthOnly);
+
+  // Display a slash
+  void DisplaySlash(nsDisplayListBuilder* aBuilder,
+                    nsIFrame* aFrame, const nsRect& aRect,
+                    nscoord aThickness,
+                    const nsDisplayListSet& aLists);
+
+  nsRect        mLineRect;
+  nsMathMLChar* mSlashChar;
+  nscoord       mLineThickness;
+  bool          mIsBevelled;
+};
+
+#endif /* nsMathMLmfracFrame_h___ */