nsMathMLmfracFrame.cpp

/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
/* vim: set ts=8 sts=2 et sw=2 tw=80: */
/* 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/. */

#include "nsMathMLmfracFrame.h"

#include "gfxUtils.h"
#include "mozilla/gfx/2D.h"
#include "mozilla/PresShell.h"
#include "mozilla/RefPtr.h"
#include "nsLayoutUtils.h"
#include "nsPresContext.h"
#include "nsDisplayList.h"
#include "gfxContext.h"
#include "nsMathMLElement.h"
#include <algorithm>
#include "gfxMathTable.h"

using namespace mozilla;
using namespace mozilla::gfx;

//
// <mfrac> -- form a fraction from two subexpressions - implementation
//

// various fraction line thicknesses (multiplicative values of the default rule
// thickness)

#define THIN_FRACTION_LINE 0.5f
#define THIN_FRACTION_LINE_MINIMUM_PIXELS 1  // minimum of 1 pixel

#define THICK_FRACTION_LINE 2.0f
#define THICK_FRACTION_LINE_MINIMUM_PIXELS 2  // minimum of 2 pixels

nsIFrame* NS_NewMathMLmfracFrame(PresShell* aPresShell, ComputedStyle* aStyle) {
  return new (aPresShell)
      nsMathMLmfracFrame(aStyle, aPresShell->GetPresContext());
}

NS_IMPL_FRAMEARENA_HELPERS(nsMathMLmfracFrame)

nsMathMLmfracFrame::~nsMathMLmfracFrame() {}

eMathMLFrameType nsMathMLmfracFrame::GetMathMLFrameType() {
  // frac is "inner" in TeXBook, Appendix G, rule 15e. See also page 170.
  return eMathMLFrameType_Inner;
}

uint8_t nsMathMLmfracFrame::ScriptIncrement(nsIFrame* aFrame) {
  if (!StyleFont()->mMathDisplay && aFrame &&
      (mFrames.FirstChild() == aFrame || mFrames.LastChild() == aFrame)) {
    return 1;
  }
  return 0;
}

NS_IMETHODIMP
nsMathMLmfracFrame::TransmitAutomaticData() {
  // The TeXbook (Ch 17. p.141) says the numerator inherits the compression
  //  while the denominator is compressed
  UpdatePresentationDataFromChildAt(1, 1, NS_MATHML_COMPRESSED,
                                    NS_MATHML_COMPRESSED);

  // If displaystyle is false, then scriptlevel is incremented, so notify the
  // children of this.
  if (!StyleFont()->mMathDisplay) {
    PropagateFrameFlagFor(mFrames.FirstChild(),
                          NS_FRAME_MATHML_SCRIPT_DESCENDANT);
    PropagateFrameFlagFor(mFrames.LastChild(),
                          NS_FRAME_MATHML_SCRIPT_DESCENDANT);
  }

  // if our numerator is an embellished operator, let its state bubble to us
  GetEmbellishDataFrom(mFrames.FirstChild(), mEmbellishData);
  if (NS_MATHML_IS_EMBELLISH_OPERATOR(mEmbellishData.flags)) {
    // even when embellished, we need to record that <mfrac> won't fire
    // Stretch() on its embellished child
    mEmbellishData.direction = NS_STRETCH_DIRECTION_UNSUPPORTED;
  }

  return NS_OK;
}

nscoord nsMathMLmfracFrame::CalcLineThickness(nsPresContext* aPresContext,
                                              ComputedStyle* aComputedStyle,
                                              nsString& aThicknessAttribute,
                                              nscoord onePixel,
                                              nscoord aDefaultRuleThickness,
                                              float aFontSizeInflation) {
  nscoord defaultThickness = aDefaultRuleThickness;
  nscoord lineThickness = aDefaultRuleThickness;
  nscoord minimumThickness = onePixel;

  // linethickness
  //
  // "Specifies the thickness of the horizontal 'fraction bar', or 'rule'. The
  // default value is 'medium', 'thin' is thinner, but visible, 'thick' is
  // thicker; the exact thickness of these is left up to the rendering agent."
  //
  // values: length | "thin" | "medium" | "thick"
  // default: medium
  //
  if (!aThicknessAttribute.IsEmpty()) {
    if (aThicknessAttribute.EqualsLiteral("thin")) {
      lineThickness = NSToCoordFloor(defaultThickness * THIN_FRACTION_LINE);
      minimumThickness = onePixel * THIN_FRACTION_LINE_MINIMUM_PIXELS;
      // should visually decrease by at least one pixel, if default is not a
      // pixel
      if (defaultThickness > onePixel &&
          lineThickness > defaultThickness - onePixel)
        lineThickness = defaultThickness - onePixel;
    } else if (aThicknessAttribute.EqualsLiteral("medium")) {
      // medium is default
    } else if (aThicknessAttribute.EqualsLiteral("thick")) {
      lineThickness = NSToCoordCeil(defaultThickness * THICK_FRACTION_LINE);
      minimumThickness = onePixel * THICK_FRACTION_LINE_MINIMUM_PIXELS;
      // should visually increase by at least one pixel
      if (lineThickness < defaultThickness + onePixel)
        lineThickness = defaultThickness + onePixel;
    } else {
      // length value
      lineThickness = defaultThickness;
      ParseNumericValue(aThicknessAttribute, &lineThickness,
                        nsMathMLElement::PARSE_ALLOW_UNITLESS, aPresContext,
                        aComputedStyle, aFontSizeInflation);
    }
  }

  // use minimum if the lineThickness is a non-zero value less than minimun
  if (lineThickness && lineThickness < minimumThickness)
    lineThickness = minimumThickness;

  return lineThickness;
}

void nsMathMLmfracFrame::BuildDisplayList(nsDisplayListBuilder* aBuilder,
                                          const nsDisplayListSet& aLists) {
  /////////////
  // paint the numerator and denominator
  nsMathMLContainerFrame::BuildDisplayList(aBuilder, aLists);

  /////////////
  // paint the fraction line
  if (mIsBevelled) {
    DisplaySlash(aBuilder, this, mLineRect, mLineThickness, aLists);
  } else {
    DisplayBar(aBuilder, this, mLineRect, aLists);
  }
}

nsresult nsMathMLmfracFrame::AttributeChanged(int32_t aNameSpaceID,
                                              nsAtom* aAttribute,
                                              int32_t aModType) {
  if (nsGkAtoms::linethickness_ == aAttribute) {
    InvalidateFrame();
  }
  return nsMathMLContainerFrame::AttributeChanged(aNameSpaceID, aAttribute,
                                                  aModType);
}

/* virtual */
nsresult nsMathMLmfracFrame::MeasureForWidth(DrawTarget* aDrawTarget,
                                             ReflowOutput& aDesiredSize) {
  return PlaceInternal(aDrawTarget, false, aDesiredSize, true);
}

nscoord nsMathMLmfracFrame::FixInterFrameSpacing(ReflowOutput& aDesiredSize) {
  nscoord gap = nsMathMLContainerFrame::FixInterFrameSpacing(aDesiredSize);
  if (!gap) return 0;

  mLineRect.MoveBy(gap, 0);
  return gap;
}

/* virtual */
nsresult nsMathMLmfracFrame::Place(DrawTarget* aDrawTarget, bool aPlaceOrigin,
                                   ReflowOutput& aDesiredSize) {
  return PlaceInternal(aDrawTarget, aPlaceOrigin, aDesiredSize, false);
}

nsresult nsMathMLmfracFrame::PlaceInternal(DrawTarget* aDrawTarget,
                                           bool aPlaceOrigin,
                                           ReflowOutput& aDesiredSize,
                                           bool aWidthOnly) {
  ////////////////////////////////////
  // Get the children's desired sizes
  nsBoundingMetrics bmNum, bmDen;
  ReflowOutput sizeNum(aDesiredSize.GetWritingMode());
  ReflowOutput sizeDen(aDesiredSize.GetWritingMode());
  nsIFrame* frameDen = nullptr;
  nsIFrame* frameNum = mFrames.FirstChild();
  if (frameNum) frameDen = frameNum->GetNextSibling();
  if (!frameNum || !frameDen || frameDen->GetNextSibling()) {
    // report an error, encourage people to get their markups in order
    if (aPlaceOrigin) {
      ReportChildCountError();
    }
    return ReflowError(aDrawTarget, aDesiredSize);
  }
  GetReflowAndBoundingMetricsFor(frameNum, sizeNum, bmNum);
  GetReflowAndBoundingMetricsFor(frameDen, sizeDen, bmDen);

  nsPresContext* presContext = PresContext();
  nscoord onePixel = nsPresContext::CSSPixelsToAppUnits(1);

  float fontSizeInflation = nsLayoutUtils::FontSizeInflationFor(this);
  RefPtr<nsFontMetrics> fm =
      nsLayoutUtils::GetFontMetricsForFrame(this, fontSizeInflation);

  nscoord defaultRuleThickness, axisHeight;
  nscoord oneDevPixel = fm->AppUnitsPerDevPixel();
  gfxFont* mathFont = fm->GetThebesFontGroup()->GetFirstMathFont();
  if (mathFont) {
    defaultRuleThickness = mathFont->MathTable()->Constant(
        gfxMathTable::FractionRuleThickness, oneDevPixel);
  } else {
    GetRuleThickness(aDrawTarget, fm, defaultRuleThickness);
  }
  GetAxisHeight(aDrawTarget, fm, axisHeight);

  bool outermostEmbellished = false;
  if (mEmbellishData.coreFrame) {
    nsEmbellishData parentData;
    GetEmbellishDataFrom(GetParent(), parentData);
    outermostEmbellished = parentData.coreFrame != mEmbellishData.coreFrame;
  }

  // see if the linethickness attribute is there
  nsAutoString value;
  mContent->AsElement()->GetAttr(kNameSpaceID_None, nsGkAtoms::linethickness_,
                                 value);
  mLineThickness =
      CalcLineThickness(presContext, mComputedStyle, value, onePixel,
                        defaultRuleThickness, fontSizeInflation);

  // bevelled attribute
  mContent->AsElement()->GetAttr(kNameSpaceID_None, nsGkAtoms::bevelled_,
                                 value);
  mIsBevelled = value.EqualsLiteral("true");

  bool displayStyle = StyleFont()->mMathDisplay == NS_MATHML_DISPLAYSTYLE_BLOCK;

  if (!mIsBevelled) {
    mLineRect.height = mLineThickness;

    // by default, leave at least one-pixel padding at either end, and add
    // lspace & rspace that may come from <mo> if we are an outermost
    // embellished container (we fetch values from the core since they may use
    // units that depend on style data, and style changes could have occurred
    // in the core since our last visit there)
    nscoord leftSpace = onePixel;
    nscoord rightSpace = onePixel;
    if (outermostEmbellished) {
      nsEmbellishData coreData;
      GetEmbellishDataFrom(mEmbellishData.coreFrame, coreData);
      leftSpace += StyleVisibility()->mDirection ? coreData.trailingSpace
                                                 : coreData.leadingSpace;
      rightSpace += StyleVisibility()->mDirection ? coreData.leadingSpace
                                                  : coreData.trailingSpace;
    }

    nscoord actualRuleThickness = mLineThickness;

    //////////////////
    // Get shifts
    nscoord numShift = 0;
    nscoord denShift = 0;

    // Rule 15b, App. G, TeXbook
    nscoord numShift1, numShift2, numShift3;
    nscoord denShift1, denShift2;

    GetNumeratorShifts(fm, numShift1, numShift2, numShift3);
    GetDenominatorShifts(fm, denShift1, denShift2);

    if (0 == actualRuleThickness) {
      numShift = displayStyle ? numShift1 : numShift3;
      denShift = displayStyle ? denShift1 : denShift2;
      if (mathFont) {
        numShift = mathFont->MathTable()->Constant(
            displayStyle ? gfxMathTable::StackTopDisplayStyleShiftUp
                         : gfxMathTable::StackTopShiftUp,
            oneDevPixel);
        denShift = mathFont->MathTable()->Constant(
            displayStyle ? gfxMathTable::StackBottomDisplayStyleShiftDown
                         : gfxMathTable::StackBottomShiftDown,
            oneDevPixel);
      }
    } else {
      numShift = displayStyle ? numShift1 : numShift2;
      denShift = displayStyle ? denShift1 : denShift2;
      if (mathFont) {
        numShift = mathFont->MathTable()->Constant(
            displayStyle ? gfxMathTable::FractionNumeratorDisplayStyleShiftUp
                         : gfxMathTable::FractionNumeratorShiftUp,
            oneDevPixel);
        denShift = mathFont->MathTable()->Constant(
            displayStyle
                ? gfxMathTable::FractionDenominatorDisplayStyleShiftDown
                : gfxMathTable::FractionDenominatorShiftDown,
            oneDevPixel);
      }
    }

    if (0 == actualRuleThickness) {
      // Rule 15c, App. G, TeXbook

      // min clearance between numerator and denominator
      nscoord minClearance =
          displayStyle ? 7 * defaultRuleThickness : 3 * defaultRuleThickness;
      if (mathFont) {
        minClearance = mathFont->MathTable()->Constant(
            displayStyle ? gfxMathTable::StackDisplayStyleGapMin
                         : gfxMathTable::StackGapMin,
            oneDevPixel);
      }
      // Factor in axis height
      // http://www.mathml-association.org/MathMLinHTML5/S3.html#SS3.SSS2
      numShift += axisHeight;
      denShift += axisHeight;

      nscoord actualClearance =
          (numShift - bmNum.descent) - (bmDen.ascent - denShift);
      // actualClearance should be >= minClearance
      if (actualClearance < minClearance) {
        nscoord halfGap = (minClearance - actualClearance) / 2;
        numShift += halfGap;
        denShift += halfGap;
      }
    } else {
      // Rule 15d, App. G, TeXbook

      // min clearance between numerator or denominator and middle of bar

      // TeX has a different interpretation of the thickness.
      // Try $a \above10pt b$ to see. Here is what TeX does:
      // minClearance = displayStyle ?
      //   3 * actualRuleThickness : actualRuleThickness;

      // we slightly depart from TeX here. We use the defaultRuleThickness
      // instead of the value coming from the linethickness attribute, i.e., we
      // recover what TeX does if the user hasn't set linethickness. But when
      // the linethickness is set, we avoid the wide gap problem.
      nscoord minClearanceNum = displayStyle ? 3 * defaultRuleThickness
                                             : defaultRuleThickness + onePixel;
      nscoord minClearanceDen = minClearanceNum;
      if (mathFont) {
        minClearanceNum = mathFont->MathTable()->Constant(
            displayStyle ? gfxMathTable::FractionNumDisplayStyleGapMin
                         : gfxMathTable::FractionNumeratorGapMin,
            oneDevPixel);
        minClearanceDen = mathFont->MathTable()->Constant(
            displayStyle ? gfxMathTable::FractionDenomDisplayStyleGapMin
                         : gfxMathTable::FractionDenominatorGapMin,
            oneDevPixel);
      }

      // adjust numShift to maintain minClearanceNum if needed
      nscoord actualClearanceNum =
          (numShift - bmNum.descent) - (axisHeight + actualRuleThickness / 2);
      if (actualClearanceNum < minClearanceNum) {
        numShift += (minClearanceNum - actualClearanceNum);
      }
      // adjust denShift to maintain minClearanceDen if needed
      nscoord actualClearanceDen =
          (axisHeight - actualRuleThickness / 2) - (bmDen.ascent - denShift);
      if (actualClearanceDen < minClearanceDen) {
        denShift += (minClearanceDen - actualClearanceDen);
      }
    }

    //////////////////
    // Place Children

    // XXX Need revisiting the width. TeX uses the exact width
    // e.g. in $$\huge\frac{\displaystyle\int}{i}$$
    nscoord width = std::max(bmNum.width, bmDen.width);
    nscoord dxNum = leftSpace + (width - sizeNum.Width()) / 2;
    nscoord dxDen = leftSpace + (width - sizeDen.Width()) / 2;
    width += leftSpace + rightSpace;

    // see if the numalign attribute is there
    mContent->AsElement()->GetAttr(kNameSpaceID_None, nsGkAtoms::numalign_,
                                   value);
    if (value.EqualsLiteral("left"))
      dxNum = leftSpace;
    else if (value.EqualsLiteral("right"))
      dxNum = width - rightSpace - sizeNum.Width();

    // see if the denomalign attribute is there
    mContent->AsElement()->GetAttr(kNameSpaceID_None, nsGkAtoms::denomalign_,
                                   value);
    if (value.EqualsLiteral("left"))
      dxDen = leftSpace;
    else if (value.EqualsLiteral("right"))
      dxDen = width - rightSpace - sizeDen.Width();

    mBoundingMetrics.rightBearing =
        std::max(dxNum + bmNum.rightBearing, dxDen + bmDen.rightBearing);
    if (mBoundingMetrics.rightBearing < width - rightSpace)
      mBoundingMetrics.rightBearing = width - rightSpace;
    mBoundingMetrics.leftBearing =
        std::min(dxNum + bmNum.leftBearing, dxDen + bmDen.leftBearing);
    if (mBoundingMetrics.leftBearing > leftSpace)
      mBoundingMetrics.leftBearing = leftSpace;
    mBoundingMetrics.ascent = bmNum.ascent + numShift;
    mBoundingMetrics.descent = bmDen.descent + denShift;
    mBoundingMetrics.width = width;

    aDesiredSize.SetBlockStartAscent(sizeNum.BlockStartAscent() + numShift);
    aDesiredSize.Height() = aDesiredSize.BlockStartAscent() + sizeDen.Height() -
                            sizeDen.BlockStartAscent() + denShift;
    aDesiredSize.Width() = mBoundingMetrics.width;
    aDesiredSize.mBoundingMetrics = mBoundingMetrics;

    mReference.x = 0;
    mReference.y = aDesiredSize.BlockStartAscent();

    if (aPlaceOrigin) {
      nscoord dy;
      // place numerator
      dy = 0;
      FinishReflowChild(frameNum, presContext, sizeNum, nullptr, dxNum, dy, 0);
      // place denominator
      dy = aDesiredSize.Height() - sizeDen.Height();
      FinishReflowChild(frameDen, presContext, sizeDen, nullptr, dxDen, dy, 0);
      // place the fraction bar - dy is top of bar
      dy = aDesiredSize.BlockStartAscent() -
           (axisHeight + actualRuleThickness / 2);
      mLineRect.SetRect(leftSpace, dy, width - (leftSpace + rightSpace),
                        actualRuleThickness);
    }
  } else {
    nscoord numShift = 0.0;
    nscoord denShift = 0.0;
    nscoord padding = 3 * defaultRuleThickness;
    nscoord slashRatio = 3;

    // Define the constant used in the expression of the maximum width
    nscoord em = fm->EmHeight();
    nscoord slashMaxWidthConstant = 2 * em;

    // For large line thicknesses the minimum slash height is limited to the
    // largest expected height of a fraction
    nscoord slashMinHeight =
        slashRatio * std::min(2 * mLineThickness, slashMaxWidthConstant);

    nscoord leadingSpace = padding;
    nscoord trailingSpace = padding;
    if (outermostEmbellished) {
      nsEmbellishData coreData;
      GetEmbellishDataFrom(mEmbellishData.coreFrame, coreData);
      leadingSpace += coreData.leadingSpace;
      trailingSpace += coreData.trailingSpace;
    }
    nscoord delta;

    //           ___________
    //          |           |    /
    //         {|-NUMERATOR-|   /
    //         {|___________|  S
    //         {               L
    // numShift{               A
    // ------------------------------------------------------- baseline
    //                         S   _____________ } denShift
    //                         H  |             |}
    //                        /   |-DENOMINATOR-|}
    //                       /    |_____________|
    //

    // first, ensure that the top of the numerator is at least as high as the
    // top of the denominator (and the reverse for the bottoms)
    delta =
        std::max(bmDen.ascent - bmNum.ascent, bmNum.descent - bmDen.descent) /
        2;
    if (delta > 0) {
      numShift += delta;
      denShift += delta;
    }

    if (StyleFont()->mMathDisplay == NS_MATHML_DISPLAYSTYLE_BLOCK) {
      delta =
          std::min(bmDen.ascent + bmDen.descent, bmNum.ascent + bmNum.descent) /
          2;
      numShift += delta;
      denShift += delta;
    } else {
      nscoord xHeight = fm->XHeight();
      numShift += xHeight / 2;
      denShift += xHeight / 4;
    }

    // Set the ascent/descent of our BoundingMetrics.
    mBoundingMetrics.ascent = bmNum.ascent + numShift;
    mBoundingMetrics.descent = bmDen.descent + denShift;

    // At this point the height of the slash is
    // mBoundingMetrics.ascent + mBoundingMetrics.descent
    // Ensure that it is greater than slashMinHeight
    delta = (slashMinHeight -
             (mBoundingMetrics.ascent + mBoundingMetrics.descent)) /
            2;
    if (delta > 0) {
      mBoundingMetrics.ascent += delta;
      mBoundingMetrics.descent += delta;
    }

    // Set the width of the slash
    if (aWidthOnly) {
      mLineRect.width = mLineThickness + slashMaxWidthConstant;
    } else {
      mLineRect.width =
          mLineThickness +
          std::min(slashMaxWidthConstant,
                   (mBoundingMetrics.ascent + mBoundingMetrics.descent) /
                       slashRatio);
    }

    // Set horizontal bounding metrics
    if (StyleVisibility()->mDirection) {
      mBoundingMetrics.leftBearing = trailingSpace + bmDen.leftBearing;
      mBoundingMetrics.rightBearing =
          trailingSpace + bmDen.width + mLineRect.width + bmNum.rightBearing;
    } else {
      mBoundingMetrics.leftBearing = leadingSpace + bmNum.leftBearing;
      mBoundingMetrics.rightBearing =
          leadingSpace + bmNum.width + mLineRect.width + bmDen.rightBearing;
    }
    mBoundingMetrics.width = leadingSpace + bmNum.width + mLineRect.width +
                             bmDen.width + trailingSpace;

    // Set aDesiredSize
    aDesiredSize.SetBlockStartAscent(mBoundingMetrics.ascent + padding);
    aDesiredSize.Height() =
        mBoundingMetrics.ascent + mBoundingMetrics.descent + 2 * padding;
    aDesiredSize.Width() = mBoundingMetrics.width;
    aDesiredSize.mBoundingMetrics = mBoundingMetrics;

    mReference.x = 0;
    mReference.y = aDesiredSize.BlockStartAscent();

    if (aPlaceOrigin) {
      nscoord dx, dy;

      // place numerator
      dx = MirrorIfRTL(aDesiredSize.Width(), sizeNum.Width(), leadingSpace);
      dy = aDesiredSize.BlockStartAscent() - numShift -
           sizeNum.BlockStartAscent();
      FinishReflowChild(frameNum, presContext, sizeNum, nullptr, dx, dy, 0);

      // place the fraction bar
      dx = MirrorIfRTL(aDesiredSize.Width(), mLineRect.width,
                       leadingSpace + bmNum.width);
      dy = aDesiredSize.BlockStartAscent() - mBoundingMetrics.ascent;
      mLineRect.SetRect(dx, dy, mLineRect.width,
                        aDesiredSize.Height() - 2 * padding);

      // place denominator
      dx = MirrorIfRTL(aDesiredSize.Width(), sizeDen.Width(),
                       leadingSpace + bmNum.width + mLineRect.width);
      dy = aDesiredSize.BlockStartAscent() + denShift -
           sizeDen.BlockStartAscent();
      FinishReflowChild(frameDen, presContext, sizeDen, nullptr, dx, dy, 0);
    }
  }

  return NS_OK;
}

class nsDisplayMathMLSlash : public nsPaintedDisplayItem {
 public:
  nsDisplayMathMLSlash(nsDisplayListBuilder* aBuilder, nsIFrame* aFrame,
                       const nsRect& aRect, nscoord aThickness, bool aRTL)
      : nsPaintedDisplayItem(aBuilder, aFrame),
        mRect(aRect),
        mThickness(aThickness),
        mRTL(aRTL) {
    MOZ_COUNT_CTOR(nsDisplayMathMLSlash);
  }
#ifdef NS_BUILD_REFCNT_LOGGING
  virtual ~nsDisplayMathMLSlash() { MOZ_COUNT_DTOR(nsDisplayMathMLSlash); }
#endif

  virtual void Paint(nsDisplayListBuilder* aBuilder, gfxContext* aCtx) override;
  NS_DISPLAY_DECL_NAME("MathMLSlash", TYPE_MATHML_SLASH)

 private:
  nsRect mRect;
  nscoord mThickness;
  bool mRTL;
};

void nsDisplayMathMLSlash::Paint(nsDisplayListBuilder* aBuilder,
                                 gfxContext* aCtx) {
  DrawTarget& aDrawTarget = *aCtx->GetDrawTarget();

  // get the gfxRect
  nsPresContext* presContext = mFrame->PresContext();
  Rect rect = NSRectToRect(mRect + ToReferenceFrame(),
                           presContext->AppUnitsPerDevPixel());

  ColorPattern color(ToDeviceColor(
      mFrame->GetVisitedDependentColor(&nsStyleText::mWebkitTextFillColor)));

  // draw the slash as a parallelogram
  Point delta = Point(presContext->AppUnitsToGfxUnits(mThickness), 0);
  RefPtr<PathBuilder> builder = aDrawTarget.CreatePathBuilder();
  if (mRTL) {
    builder->MoveTo(rect.TopLeft());
    builder->LineTo(rect.TopLeft() + delta);
    builder->LineTo(rect.BottomRight());
    builder->LineTo(rect.BottomRight() - delta);
  } else {
    builder->MoveTo(rect.BottomLeft());
    builder->LineTo(rect.BottomLeft() + delta);
    builder->LineTo(rect.TopRight());
    builder->LineTo(rect.TopRight() - delta);
  }
  RefPtr<Path> path = builder->Finish();
  aDrawTarget.Fill(path, color);
}

void nsMathMLmfracFrame::DisplaySlash(nsDisplayListBuilder* aBuilder,
                                      nsIFrame* aFrame, const nsRect& aRect,
                                      nscoord aThickness,
                                      const nsDisplayListSet& aLists) {
  if (!aFrame->StyleVisibility()->IsVisible() || aRect.IsEmpty()) return;

  aLists.Content()->AppendNewToTop<nsDisplayMathMLSlash>(
      aBuilder, aFrame, aRect, aThickness, StyleVisibility()->mDirection);
}