Adding upstream version 124.0.1.upstream/124.0.1

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 697 insertions, 0 deletions

diff --git a/layout/mathml/nsMathMLmunderoverFrame.cpp b/layout/mathml/nsMathMLmunderoverFrame.cpp
new file mode 100644
index 0000000000..05bf1d2886
--- /dev/null
+++ b/layout/mathml/nsMathMLmunderoverFrame.cpp
@@ -0,0 +1,697 @@
+/* -*- 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 "nsMathMLmunderoverFrame.h"
+#include "nsLayoutUtils.h"
+#include "nsPresContext.h"
+#include "nsMathMLmmultiscriptsFrame.h"
+#include "mozilla/dom/Document.h"
+#include "mozilla/dom/MathMLElement.h"
+#include <algorithm>
+#include "gfxContext.h"
+#include "gfxMathTable.h"
+#include "gfxTextRun.h"
+#include "mozilla/PresShell.h"
+#include "mozilla/StaticPrefs_mathml.h"
+
+using namespace mozilla;
+
+//
+// <munderover> -- attach an underscript-overscript pair to a base
+//                 implementation
+// <mover> -- attach an overscript to a base - implementation
+// <munder> -- attach an underscript to a base - implementation
+//
+
+nsIFrame* NS_NewMathMLmunderoverFrame(PresShell* aPresShell,
+                                      ComputedStyle* aStyle) {
+  return new (aPresShell)
+      nsMathMLmunderoverFrame(aStyle, aPresShell->GetPresContext());
+}
+
+NS_IMPL_FRAMEARENA_HELPERS(nsMathMLmunderoverFrame)
+
+nsMathMLmunderoverFrame::~nsMathMLmunderoverFrame() = default;
+
+nsresult nsMathMLmunderoverFrame::AttributeChanged(int32_t aNameSpaceID,
+                                                   nsAtom* aAttribute,
+                                                   int32_t aModType) {
+  if (nsGkAtoms::accent_ == aAttribute ||
+      nsGkAtoms::accentunder_ == aAttribute) {
+    // When we have automatic data to update within ourselves, we ask our
+    // parent to re-layout its children
+    return ReLayoutChildren(GetParent());
+  }
+
+  return nsMathMLContainerFrame::AttributeChanged(aNameSpaceID, aAttribute,
+                                                  aModType);
+}
+
+NS_IMETHODIMP
+nsMathMLmunderoverFrame::UpdatePresentationData(uint32_t aFlagsValues,
+                                                uint32_t aFlagsToUpdate) {
+  nsMathMLContainerFrame::UpdatePresentationData(aFlagsValues, aFlagsToUpdate);
+  // disable the stretch-all flag if we are going to act like a
+  // subscript-superscript pair
+  if (NS_MATHML_EMBELLISH_IS_MOVABLELIMITS(mEmbellishData.flags) &&
+      StyleFont()->mMathStyle == StyleMathStyle::Compact) {
+    mPresentationData.flags &= ~NS_MATHML_STRETCH_ALL_CHILDREN_HORIZONTALLY;
+  } else {
+    mPresentationData.flags |= NS_MATHML_STRETCH_ALL_CHILDREN_HORIZONTALLY;
+  }
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsMathMLmunderoverFrame::InheritAutomaticData(nsIFrame* aParent) {
+  // let the base class get the default from our parent
+  nsMathMLContainerFrame::InheritAutomaticData(aParent);
+
+  mPresentationData.flags |= NS_MATHML_STRETCH_ALL_CHILDREN_HORIZONTALLY;
+
+  return NS_OK;
+}
+
+void nsMathMLmunderoverFrame::Destroy(DestroyContext& aContext) {
+  if (!mPostReflowIncrementScriptLevelCommands.IsEmpty()) {
+    PresShell()->CancelReflowCallback(this);
+  }
+  nsMathMLContainerFrame::Destroy(aContext);
+}
+
+uint8_t nsMathMLmunderoverFrame::ScriptIncrement(nsIFrame* aFrame) {
+  nsIFrame* child = mFrames.FirstChild();
+  if (!aFrame || aFrame == child) {
+    return 0;
+  }
+  child = child->GetNextSibling();
+  if (aFrame == child) {
+    if (mContent->IsMathMLElement(nsGkAtoms::mover_)) {
+      return mIncrementOver ? 1 : 0;
+    }
+    return mIncrementUnder ? 1 : 0;
+  }
+  if (child && aFrame == child->GetNextSibling()) {
+    // must be a over frame of munderover
+    return mIncrementOver ? 1 : 0;
+  }
+  return 0;  // frame not found
+}
+
+void nsMathMLmunderoverFrame::SetIncrementScriptLevel(uint32_t aChildIndex,
+                                                      bool aIncrement) {
+  nsIFrame* child = PrincipalChildList().FrameAt(aChildIndex);
+  if (!child || !child->GetContent()->IsMathMLElement() ||
+      child->GetContent()->GetPrimaryFrame() != child) {
+    return;
+  }
+
+  auto element = dom::MathMLElement::FromNode(child->GetContent());
+  if (element->GetIncrementScriptLevel() == aIncrement) {
+    return;
+  }
+
+  if (mPostReflowIncrementScriptLevelCommands.IsEmpty()) {
+    PresShell()->PostReflowCallback(this);
+  }
+
+  mPostReflowIncrementScriptLevelCommands.AppendElement(
+      SetIncrementScriptLevelCommand{aChildIndex, aIncrement});
+}
+
+bool nsMathMLmunderoverFrame::ReflowFinished() {
+  SetPendingPostReflowIncrementScriptLevel();
+  return true;
+}
+
+void nsMathMLmunderoverFrame::ReflowCallbackCanceled() {
+  // Do nothing, at this point our work will just be useless.
+  mPostReflowIncrementScriptLevelCommands.Clear();
+}
+
+void nsMathMLmunderoverFrame::SetPendingPostReflowIncrementScriptLevel() {
+  MOZ_ASSERT(!mPostReflowIncrementScriptLevelCommands.IsEmpty());
+
+  nsTArray<SetIncrementScriptLevelCommand> commands =
+      std::move(mPostReflowIncrementScriptLevelCommands);
+
+  for (const auto& command : commands) {
+    nsIFrame* child = PrincipalChildList().FrameAt(command.mChildIndex);
+    if (!child || !child->GetContent()->IsMathMLElement()) {
+      continue;
+    }
+
+    auto element = dom::MathMLElement::FromNode(child->GetContent());
+    element->SetIncrementScriptLevel(command.mDoIncrement, true);
+  }
+}
+
+NS_IMETHODIMP
+nsMathMLmunderoverFrame::TransmitAutomaticData() {
+  // At this stage, all our children are in sync and we can fully
+  // resolve our own mEmbellishData struct
+  //---------------------------------------------------------------------
+
+  /*
+  The REC says:
+
+  As regards munder (respectively mover) :
+  The default value of accentunder is false, unless underscript
+  is an <mo> element or an embellished operator.  If underscript is
+  an <mo> element, the value of its accent attribute is used as the
+  default value of accentunder. If underscript is an embellished
+  operator, the accent attribute of the <mo> element at its
+  core is used as the default value. As with all attributes, an
+  explicitly given value overrides the default.
+
+XXX The winner is the outermost setting in conflicting settings like these:
+<munder accentunder='true'>
+  <mi>...</mi>
+  <mo accentunder='false'> ... </mo>
+</munder>
+
+  As regards munderover:
+  The accent and accentunder attributes have the same effect as
+  the attributes with the same names on <mover>  and <munder>,
+  respectively. Their default values are also computed in the
+  same manner as described for those elements, with the default
+  value of accent depending on overscript and the default value
+  of accentunder depending on underscript.
+  */
+
+  nsIFrame* overscriptFrame = nullptr;
+  nsIFrame* underscriptFrame = nullptr;
+  nsIFrame* baseFrame = mFrames.FirstChild();
+
+  if (baseFrame) {
+    if (mContent->IsAnyOfMathMLElements(nsGkAtoms::munder_,
+                                        nsGkAtoms::munderover_)) {
+      underscriptFrame = baseFrame->GetNextSibling();
+    } else {
+      NS_ASSERTION(mContent->IsMathMLElement(nsGkAtoms::mover_),
+                   "mContent->NodeInfo()->NameAtom() not recognized");
+      overscriptFrame = baseFrame->GetNextSibling();
+    }
+  }
+  if (underscriptFrame && mContent->IsMathMLElement(nsGkAtoms::munderover_)) {
+    overscriptFrame = underscriptFrame->GetNextSibling();
+  }
+
+  // if our base is an embellished operator, let its state bubble to us (in
+  // particular, this is where we get the flag for
+  // NS_MATHML_EMBELLISH_MOVABLELIMITS). Our flags are reset to the default
+  // values of false if the base frame isn't embellished.
+  mPresentationData.baseFrame = baseFrame;
+  GetEmbellishDataFrom(baseFrame, mEmbellishData);
+
+  // The default value of accentunder is false, unless the underscript is
+  // embellished and its core <mo> is an accent
+  nsEmbellishData embellishData;
+  nsAutoString value;
+  if (mContent->IsAnyOfMathMLElements(nsGkAtoms::munder_,
+                                      nsGkAtoms::munderover_)) {
+    GetEmbellishDataFrom(underscriptFrame, embellishData);
+    if (NS_MATHML_EMBELLISH_IS_ACCENT(embellishData.flags)) {
+      mEmbellishData.flags |= NS_MATHML_EMBELLISH_ACCENTUNDER;
+    } else {
+      mEmbellishData.flags &= ~NS_MATHML_EMBELLISH_ACCENTUNDER;
+    }
+
+    // if we have an accentunder attribute, it overrides what the underscript
+    // said
+    if (mContent->AsElement()->GetAttr(nsGkAtoms::accentunder_, value)) {
+      if (value.EqualsLiteral("true")) {
+        mEmbellishData.flags |= NS_MATHML_EMBELLISH_ACCENTUNDER;
+      } else if (value.EqualsLiteral("false")) {
+        mEmbellishData.flags &= ~NS_MATHML_EMBELLISH_ACCENTUNDER;
+      }
+    }
+  }
+
+  // The default value of accent is false, unless the overscript is embellished
+  // and its core <mo> is an accent
+  if (mContent->IsAnyOfMathMLElements(nsGkAtoms::mover_,
+                                      nsGkAtoms::munderover_)) {
+    GetEmbellishDataFrom(overscriptFrame, embellishData);
+    if (NS_MATHML_EMBELLISH_IS_ACCENT(embellishData.flags)) {
+      mEmbellishData.flags |= NS_MATHML_EMBELLISH_ACCENTOVER;
+    } else {
+      mEmbellishData.flags &= ~NS_MATHML_EMBELLISH_ACCENTOVER;
+    }
+
+    // if we have an accent attribute, it overrides what the overscript said
+    if (mContent->AsElement()->GetAttr(nsGkAtoms::accent_, value)) {
+      if (value.EqualsLiteral("true")) {
+        mEmbellishData.flags |= NS_MATHML_EMBELLISH_ACCENTOVER;
+      } else if (value.EqualsLiteral("false")) {
+        mEmbellishData.flags &= ~NS_MATHML_EMBELLISH_ACCENTOVER;
+      }
+    }
+  }
+
+  bool subsupDisplay =
+      NS_MATHML_EMBELLISH_IS_MOVABLELIMITS(mEmbellishData.flags) &&
+      StyleFont()->mMathStyle == StyleMathStyle::Compact;
+
+  // disable the stretch-all flag if we are going to act like a superscript
+  if (subsupDisplay) {
+    mPresentationData.flags &= ~NS_MATHML_STRETCH_ALL_CHILDREN_HORIZONTALLY;
+  }
+
+  // Now transmit any change that we want to our children so that they
+  // can update their mPresentationData structs
+  //---------------------------------------------------------------------
+
+  /* The REC says:
+     Within underscript, <munderover> always sets displaystyle to "false",
+     but increments scriptlevel by 1 only when accentunder is "false".
+
+     Within overscript, <munderover> always sets displaystyle to "false",
+     but increments scriptlevel by 1 only when accent is "false".
+
+     Within subscript and superscript it increments scriptlevel by 1, and
+     sets displaystyle to "false", but leaves both attributes unchanged within
+     base.
+
+     The TeXBook treats 'over' like a superscript, so p.141 or Rule 13a
+     say it shouldn't be compressed. However, The TeXBook says
+     that math accents and \overline change uncramped styles to their
+     cramped counterparts.
+  */
+  if (mContent->IsAnyOfMathMLElements(nsGkAtoms::mover_,
+                                      nsGkAtoms::munderover_)) {
+    uint32_t compress = NS_MATHML_EMBELLISH_IS_ACCENTOVER(mEmbellishData.flags)
+                            ? NS_MATHML_COMPRESSED
+                            : 0;
+    mIncrementOver = !NS_MATHML_EMBELLISH_IS_ACCENTOVER(mEmbellishData.flags) ||
+                     subsupDisplay;
+    SetIncrementScriptLevel(
+        mContent->IsMathMLElement(nsGkAtoms::mover_) ? 1 : 2, mIncrementOver);
+    if (mIncrementOver) {
+      PropagateFrameFlagFor(overscriptFrame, NS_FRAME_MATHML_SCRIPT_DESCENDANT);
+    }
+    PropagatePresentationDataFor(overscriptFrame, compress, compress);
+  }
+  /*
+     The TeXBook treats 'under' like a subscript, so p.141 or Rule 13a
+     say it should be compressed
+  */
+  if (mContent->IsAnyOfMathMLElements(nsGkAtoms::munder_,
+                                      nsGkAtoms::munderover_)) {
+    mIncrementUnder =
+        !NS_MATHML_EMBELLISH_IS_ACCENTUNDER(mEmbellishData.flags) ||
+        subsupDisplay;
+    SetIncrementScriptLevel(1, mIncrementUnder);
+    if (mIncrementUnder) {
+      PropagateFrameFlagFor(underscriptFrame,
+                            NS_FRAME_MATHML_SCRIPT_DESCENDANT);
+    }
+    PropagatePresentationDataFor(underscriptFrame, NS_MATHML_COMPRESSED,
+                                 NS_MATHML_COMPRESSED);
+  }
+
+  /* Set flags for dtls font feature settings.
+
+     dtls
+     Dotless Forms
+     This feature provides dotless forms for Math Alphanumeric
+     characters, such as U+1D422 MATHEMATICAL BOLD SMALL I,
+     U+1D423 MATHEMATICAL BOLD SMALL J, U+1D456
+     U+MATHEMATICAL ITALIC SMALL I, U+1D457 MATHEMATICAL ITALIC
+     SMALL J, and so on.
+     The dotless forms are to be used as base forms for placing
+     mathematical accents over them.
+
+     To opt out of this change, add the following to the stylesheet:
+     "font-feature-settings: 'dtls' 0"
+   */
+  if (overscriptFrame &&
+      NS_MATHML_EMBELLISH_IS_ACCENTOVER(mEmbellishData.flags) &&
+      !NS_MATHML_EMBELLISH_IS_MOVABLELIMITS(mEmbellishData.flags)) {
+    PropagatePresentationDataFor(baseFrame, NS_MATHML_DTLS, NS_MATHML_DTLS);
+  }
+
+  return NS_OK;
+}
+
+/*
+The REC says:
+*  If the base is an operator with movablelimits="true" (or an embellished
+   operator whose <mo> element core has movablelimits="true"), and
+   displaystyle="false", then underscript and overscript are drawn in
+   a subscript and superscript position, respectively. In this case,
+   the accent and accentunder attributes are ignored. This is often
+   used for limits on symbols such as &sum;.
+
+i.e.,:
+ if (NS_MATHML_EMBELLISH_IS_MOVABLELIMITS(mEmbellishDataflags) &&
+     StyleFont()->mMathStyle == StyleMathStyle::Compact) {
+  // place like subscript-superscript pair
+ }
+ else {
+  // place like underscript-overscript pair
+ }
+*/
+
+/* virtual */
+nsresult nsMathMLmunderoverFrame::Place(DrawTarget* aDrawTarget,
+                                        bool aPlaceOrigin,
+                                        ReflowOutput& aDesiredSize) {
+  float fontSizeInflation = nsLayoutUtils::FontSizeInflationFor(this);
+  if (NS_MATHML_EMBELLISH_IS_MOVABLELIMITS(mEmbellishData.flags) &&
+      StyleFont()->mMathStyle == StyleMathStyle::Compact) {
+    // place like sub sup or subsup
+    if (mContent->IsMathMLElement(nsGkAtoms::munderover_)) {
+      return nsMathMLmmultiscriptsFrame::PlaceMultiScript(
+          PresContext(), aDrawTarget, aPlaceOrigin, aDesiredSize, this, 0, 0,
+          fontSizeInflation);
+    } else if (mContent->IsMathMLElement(nsGkAtoms::munder_)) {
+      return nsMathMLmmultiscriptsFrame::PlaceMultiScript(
+          PresContext(), aDrawTarget, aPlaceOrigin, aDesiredSize, this, 0, 0,
+          fontSizeInflation);
+    } else {
+      NS_ASSERTION(mContent->IsMathMLElement(nsGkAtoms::mover_),
+                   "mContent->NodeInfo()->NameAtom() not recognized");
+      return nsMathMLmmultiscriptsFrame::PlaceMultiScript(
+          PresContext(), aDrawTarget, aPlaceOrigin, aDesiredSize, this, 0, 0,
+          fontSizeInflation);
+    }
+  }
+
+  ////////////////////////////////////
+  // Get the children's desired sizes
+
+  nsBoundingMetrics bmBase, bmUnder, bmOver;
+  ReflowOutput baseSize(aDesiredSize.GetWritingMode());
+  ReflowOutput underSize(aDesiredSize.GetWritingMode());
+  ReflowOutput overSize(aDesiredSize.GetWritingMode());
+  nsIFrame* overFrame = nullptr;
+  nsIFrame* underFrame = nullptr;
+  nsIFrame* baseFrame = mFrames.FirstChild();
+  underSize.SetBlockStartAscent(0);
+  overSize.SetBlockStartAscent(0);
+  bool haveError = false;
+  if (baseFrame) {
+    if (mContent->IsAnyOfMathMLElements(nsGkAtoms::munder_,
+                                        nsGkAtoms::munderover_)) {
+      underFrame = baseFrame->GetNextSibling();
+    } else if (mContent->IsMathMLElement(nsGkAtoms::mover_)) {
+      overFrame = baseFrame->GetNextSibling();
+    }
+  }
+  if (underFrame && mContent->IsMathMLElement(nsGkAtoms::munderover_)) {
+    overFrame = underFrame->GetNextSibling();
+  }
+
+  if (mContent->IsMathMLElement(nsGkAtoms::munder_)) {
+    if (!baseFrame || !underFrame || underFrame->GetNextSibling()) {
+      // report an error, encourage people to get their markups in order
+      haveError = true;
+    }
+  }
+  if (mContent->IsMathMLElement(nsGkAtoms::mover_)) {
+    if (!baseFrame || !overFrame || overFrame->GetNextSibling()) {
+      // report an error, encourage people to get their markups in order
+      haveError = true;
+    }
+  }
+  if (mContent->IsMathMLElement(nsGkAtoms::munderover_)) {
+    if (!baseFrame || !underFrame || !overFrame ||
+        overFrame->GetNextSibling()) {
+      // report an error, encourage people to get their markups in order
+      haveError = true;
+    }
+  }
+  if (haveError) {
+    if (aPlaceOrigin) {
+      ReportChildCountError();
+    }
+    return PlaceAsMrow(aDrawTarget, aPlaceOrigin, aDesiredSize);
+  }
+  GetReflowAndBoundingMetricsFor(baseFrame, baseSize, bmBase);
+  if (underFrame) {
+    GetReflowAndBoundingMetricsFor(underFrame, underSize, bmUnder);
+  }
+  if (overFrame) {
+    GetReflowAndBoundingMetricsFor(overFrame, overSize, bmOver);
+  }
+
+  nscoord onePixel = nsPresContext::CSSPixelsToAppUnits(1);
+
+  ////////////////////
+  // Place Children
+
+  RefPtr<nsFontMetrics> fm =
+      nsLayoutUtils::GetFontMetricsForFrame(this, fontSizeInflation);
+
+  nscoord xHeight = fm->XHeight();
+  nscoord oneDevPixel = fm->AppUnitsPerDevPixel();
+  RefPtr<gfxFont> mathFont = fm->GetThebesFontGroup()->GetFirstMathFont();
+
+  nscoord ruleThickness;
+  GetRuleThickness(aDrawTarget, fm, ruleThickness);
+
+  nscoord correction = 0;
+  GetItalicCorrection(bmBase, correction);
+
+  // there are 2 different types of placement depending on
+  // whether we want an accented under or not
+
+  nscoord underDelta1 = 0;  // gap between base and underscript
+  nscoord underDelta2 = 0;  // extra space beneath underscript
+
+  if (!NS_MATHML_EMBELLISH_IS_ACCENTUNDER(mEmbellishData.flags)) {
+    // Rule 13a, App. G, TeXbook
+    nscoord bigOpSpacing2, bigOpSpacing4, bigOpSpacing5, dummy;
+    GetBigOpSpacings(fm, dummy, bigOpSpacing2, dummy, bigOpSpacing4,
+                     bigOpSpacing5);
+    if (mathFont) {
+      // XXXfredw The Open Type MATH table has some StretchStack* parameters
+      // that we may use when the base is a stretchy horizontal operator. See
+      // bug 963131.
+      bigOpSpacing2 = mathFont->MathTable()->Constant(
+          gfxMathTable::LowerLimitGapMin, oneDevPixel);
+      bigOpSpacing4 = mathFont->MathTable()->Constant(
+          gfxMathTable::LowerLimitBaselineDropMin, oneDevPixel);
+      bigOpSpacing5 = 0;
+    }
+    underDelta1 = std::max(bigOpSpacing2, (bigOpSpacing4 - bmUnder.ascent));
+    underDelta2 = bigOpSpacing5;
+  } else {
+    // No corresponding rule in TeXbook - we are on our own here
+    // XXX tune the gap delta between base and underscript
+    // XXX Should we use Rule 10 like \underline does?
+    // XXXfredw Perhaps use the Underbar* parameters of the MATH table. See
+    // bug 963125.
+    underDelta1 = ruleThickness + onePixel / 2;
+    underDelta2 = ruleThickness;
+  }
+  // empty under?
+  if (!(bmUnder.ascent + bmUnder.descent)) {
+    underDelta1 = 0;
+    underDelta2 = 0;
+  }
+
+  nscoord overDelta1 = 0;  // gap between base and overscript
+  nscoord overDelta2 = 0;  // extra space above overscript
+
+  if (!NS_MATHML_EMBELLISH_IS_ACCENTOVER(mEmbellishData.flags)) {
+    // Rule 13a, App. G, TeXbook
+    // XXXfredw The Open Type MATH table has some StretchStack* parameters
+    // that we may use when the base is a stretchy horizontal operator. See
+    // bug 963131.
+    nscoord bigOpSpacing1, bigOpSpacing3, bigOpSpacing5, dummy;
+    GetBigOpSpacings(fm, bigOpSpacing1, dummy, bigOpSpacing3, dummy,
+                     bigOpSpacing5);
+    if (mathFont) {
+      // XXXfredw The Open Type MATH table has some StretchStack* parameters
+      // that we may use when the base is a stretchy horizontal operator. See
+      // bug 963131.
+      bigOpSpacing1 = mathFont->MathTable()->Constant(
+          gfxMathTable::UpperLimitGapMin, oneDevPixel);
+      bigOpSpacing3 = mathFont->MathTable()->Constant(
+          gfxMathTable::UpperLimitBaselineRiseMin, oneDevPixel);
+      bigOpSpacing5 = 0;
+    }
+    overDelta1 = std::max(bigOpSpacing1, (bigOpSpacing3 - bmOver.descent));
+    overDelta2 = bigOpSpacing5;
+
+    // XXX This is not a TeX rule...
+    // delta1 (as computed abvove) can become really big when bmOver.descent is
+    // negative,  e.g., if the content is &OverBar. In such case, we use the
+    // height
+    if (bmOver.descent < 0)
+      overDelta1 = std::max(bigOpSpacing1,
+                            (bigOpSpacing3 - (bmOver.ascent + bmOver.descent)));
+  } else {
+    // Rule 12, App. G, TeXbook
+    // We are going to modify this rule to make it more general.
+    // The idea behind Rule 12 in the TeXBook is to keep the accent
+    // as close to the base as possible, while ensuring that the
+    // distance between the *baseline* of the accent char and
+    // the *baseline* of the base is atleast x-height.
+    // The idea is that for normal use, we would like all the accents
+    // on a line to line up atleast x-height above the baseline
+    // if possible.
+    // When the ascent of the base is >= x-height,
+    // the baseline of the accent char is placed just above the base
+    // (specifically, the baseline of the accent char is placed
+    // above the baseline of the base by the ascent of the base).
+    // For ease of implementation,
+    // this assumes that the font-designer designs accents
+    // in such a way that the bottom of the accent is atleast x-height
+    // above its baseline, otherwise there will be collisions
+    // with the base. Also there should be proper padding between
+    // the bottom of the accent char and its baseline.
+    // The above rule may not be obvious from a first
+    // reading of rule 12 in the TeXBook !!!
+    // The mathml <mover> tag can use accent chars that
+    // do not follow this convention. So we modify TeX's rule
+    // so that TeX's rule gets subsumed for accents that follow
+    // TeX's convention,
+    // while also allowing accents that do not follow the convention :
+    // we try to keep the *bottom* of the accent char atleast x-height
+    // from the baseline of the base char. we also slap on an extra
+    // padding between the accent and base chars.
+    overDelta1 = ruleThickness + onePixel / 2;
+    nscoord accentBaseHeight = xHeight;
+    if (mathFont) {
+      accentBaseHeight = mathFont->MathTable()->Constant(
+          gfxMathTable::AccentBaseHeight, oneDevPixel);
+    }
+    if (bmBase.ascent < accentBaseHeight) {
+      // also ensure at least accentBaseHeight above the baseline of the base
+      overDelta1 += accentBaseHeight - bmBase.ascent;
+    }
+    overDelta2 = ruleThickness;
+  }
+  // empty over?
+  if (!(bmOver.ascent + bmOver.descent)) {
+    overDelta1 = 0;
+    overDelta2 = 0;
+  }
+
+  nscoord dxBase = 0, dxOver = 0, dxUnder = 0;
+  nsAutoString valueAlign;
+
+  //////////
+  // pass 1, do what <mover> does: attach the overscript on the base
+
+  // Ad-hoc - This is to override fonts which have ready-made _accent_
+  // glyphs with negative lbearing and rbearing. We want to position
+  // the overscript ourselves
+  nscoord overWidth = bmOver.width;
+  if (!overWidth && (bmOver.rightBearing - bmOver.leftBearing > 0)) {
+    overWidth = bmOver.rightBearing - bmOver.leftBearing;
+    dxOver = -bmOver.leftBearing;
+  }
+
+  if (NS_MATHML_EMBELLISH_IS_ACCENTOVER(mEmbellishData.flags)) {
+    mBoundingMetrics.width = bmBase.width;
+    dxOver += correction;
+  } else {
+    mBoundingMetrics.width = std::max(bmBase.width, overWidth);
+    dxOver += correction / 2;
+  }
+
+  dxOver += (mBoundingMetrics.width - overWidth) / 2;
+  dxBase = (mBoundingMetrics.width - bmBase.width) / 2;
+
+  mBoundingMetrics.ascent =
+      bmBase.ascent + overDelta1 + bmOver.ascent + bmOver.descent;
+  mBoundingMetrics.descent = bmBase.descent;
+  mBoundingMetrics.leftBearing =
+      std::min(dxBase + bmBase.leftBearing, dxOver + bmOver.leftBearing);
+  mBoundingMetrics.rightBearing =
+      std::max(dxBase + bmBase.rightBearing, dxOver + bmOver.rightBearing);
+
+  //////////
+  // pass 2, do what <munder> does: attach the underscript on the previous
+  // result. We conceptually view the previous result as an "anynomous base"
+  // from where to attach the underscript. Hence if the underscript is empty,
+  // we should end up like <mover>. If the overscript is empty, we should
+  // end up like <munder>.
+
+  nsBoundingMetrics bmAnonymousBase = mBoundingMetrics;
+  nscoord ascentAnonymousBase =
+      std::max(mBoundingMetrics.ascent + overDelta2,
+               overSize.BlockStartAscent() + bmOver.descent + overDelta1 +
+                   bmBase.ascent);
+  ascentAnonymousBase =
+      std::max(ascentAnonymousBase, baseSize.BlockStartAscent());
+
+  // Width of non-spacing marks is zero so use left and right bearing.
+  nscoord underWidth = bmUnder.width;
+  if (!underWidth) {
+    underWidth = bmUnder.rightBearing - bmUnder.leftBearing;
+    dxUnder = -bmUnder.leftBearing;
+  }
+
+  nscoord maxWidth = std::max(bmAnonymousBase.width, underWidth);
+  if (!NS_MATHML_EMBELLISH_IS_ACCENTUNDER(mEmbellishData.flags)) {
+    GetItalicCorrection(bmAnonymousBase, correction);
+    dxUnder += -correction / 2;
+  }
+  nscoord dxAnonymousBase = 0;
+  dxUnder += (maxWidth - underWidth) / 2;
+  dxAnonymousBase = (maxWidth - bmAnonymousBase.width) / 2;
+
+  // adjust the offsets of the real base and overscript since their
+  // final offsets should be relative to us...
+  dxOver += dxAnonymousBase;
+  dxBase += dxAnonymousBase;
+
+  mBoundingMetrics.width = std::max(dxAnonymousBase + bmAnonymousBase.width,
+                                    dxUnder + bmUnder.width);
+  // At this point, mBoundingMetrics.ascent = bmAnonymousBase.ascent
+  mBoundingMetrics.descent =
+      bmAnonymousBase.descent + underDelta1 + bmUnder.ascent + bmUnder.descent;
+  mBoundingMetrics.leftBearing =
+      std::min(dxAnonymousBase + bmAnonymousBase.leftBearing,
+               dxUnder + bmUnder.leftBearing);
+  mBoundingMetrics.rightBearing =
+      std::max(dxAnonymousBase + bmAnonymousBase.rightBearing,
+               dxUnder + bmUnder.rightBearing);
+
+  aDesiredSize.SetBlockStartAscent(ascentAnonymousBase);
+  aDesiredSize.Height() =
+      aDesiredSize.BlockStartAscent() +
+      std::max(mBoundingMetrics.descent + underDelta2,
+               bmAnonymousBase.descent + underDelta1 + bmUnder.ascent +
+                   underSize.Height() - underSize.BlockStartAscent());
+  aDesiredSize.Height() =
+      std::max(aDesiredSize.Height(), aDesiredSize.BlockStartAscent() +
+                                          baseSize.Height() -
+                                          baseSize.BlockStartAscent());
+  aDesiredSize.Width() = mBoundingMetrics.width;
+  aDesiredSize.mBoundingMetrics = mBoundingMetrics;
+
+  mReference.x = 0;
+  mReference.y = aDesiredSize.BlockStartAscent();
+
+  if (aPlaceOrigin) {
+    nscoord dy;
+    // place overscript
+    if (overFrame) {
+      dy = aDesiredSize.BlockStartAscent() - mBoundingMetrics.ascent +
+           bmOver.ascent - overSize.BlockStartAscent();
+      FinishReflowChild(overFrame, PresContext(), overSize, nullptr, dxOver, dy,
+                        ReflowChildFlags::Default);
+    }
+    // place base
+    dy = aDesiredSize.BlockStartAscent() - baseSize.BlockStartAscent();
+    FinishReflowChild(baseFrame, PresContext(), baseSize, nullptr, dxBase, dy,
+                      ReflowChildFlags::Default);
+    // place underscript
+    if (underFrame) {
+      dy = aDesiredSize.BlockStartAscent() + mBoundingMetrics.descent -
+           bmUnder.descent - underSize.BlockStartAscent();
+      FinishReflowChild(underFrame, PresContext(), underSize, nullptr, dxUnder,
+                        dy, ReflowChildFlags::Default);
+    }
+  }
+  return NS_OK;
+}