Bug 1414926 - Make -moz-font-smoothing-background-color changes only cause repaints, not reflows. r=emilio

This case is hit by hovering over menu items, so the optimization is somewhat worthwhile.
As a side-effect, not causing reflows also avoids a XUL <select> popup positioning bug.

MozReview-Commit-ID: AOrijytoHHL

--HG--
extra : rebase_source : c2156eb24171f6d0b0e5476c4a7dbc641a0b5301

gfx/src/nsFont.cpp

@@ -42,32 +42,42 @@ nsFont::~nsFont()
 
 bool nsFont::Equals(const nsFont& aOther) const
 {
-  if ((style == aOther.style) &&
+  return CalcDifference(aOther) == MaxDifference::eNone;
-      (systemFont == aOther.systemFont) &&
+}
-      (weight == aOther.weight) &&
+
-      (stretch == aOther.stretch) &&
+nsFont::MaxDifference
-      (size == aOther.size) &&
+nsFont::CalcDifference(const nsFont& aOther) const
-      (sizeAdjust == aOther.sizeAdjust) &&
+{
-      (fontlist == aOther.fontlist) &&
+  if ((style != aOther.style) ||
-      (kerning == aOther.kerning) &&
+      (systemFont != aOther.systemFont) ||
-      (synthesis == aOther.synthesis) &&
+      (weight != aOther.weight) ||
-      (fontFeatureSettings == aOther.fontFeatureSettings) &&
+      (stretch != aOther.stretch) ||
-      (fontVariationSettings == aOther.fontVariationSettings) &&
+      (size != aOther.size) ||
-      (languageOverride == aOther.languageOverride) &&
+      (sizeAdjust != aOther.sizeAdjust) ||
-      (variantAlternates == aOther.variantAlternates) &&
+      (fontlist != aOther.fontlist) ||
-      (variantCaps == aOther.variantCaps) &&
+      (kerning != aOther.kerning) ||
-      (variantEastAsian == aOther.variantEastAsian) &&
+      (synthesis != aOther.synthesis) ||
-      (variantLigatures == aOther.variantLigatures) &&
+      (fontFeatureSettings != aOther.fontFeatureSettings) ||
-      (variantNumeric == aOther.variantNumeric) &&
+      (fontVariationSettings != aOther.fontVariationSettings) ||
-      (variantPosition == aOther.variantPosition) &&
+      (languageOverride != aOther.languageOverride) ||
-      (variantWidth == aOther.variantWidth) &&
+      (variantAlternates != aOther.variantAlternates) ||
-      (alternateValues == aOther.alternateValues) &&
+      (variantCaps != aOther.variantCaps) ||
-      (featureValueLookup == aOther.featureValueLookup) &&
+      (variantEastAsian != aOther.variantEastAsian) ||
-      (smoothing == aOther.smoothing) &&
+      (variantLigatures != aOther.variantLigatures) ||
-      (fontSmoothingBackgroundColor == aOther.fontSmoothingBackgroundColor)) {
+      (variantNumeric != aOther.variantNumeric) ||
-    return true;
+      (variantPosition != aOther.variantPosition) ||
+      (variantWidth != aOther.variantWidth) ||
+      (alternateValues != aOther.alternateValues) ||
+      (featureValueLookup != aOther.featureValueLookup)) {
+    return MaxDifference::eLayoutAffecting;
   }
-  return false;
+
+  if ((smoothing != aOther.smoothing) ||
+      (fontSmoothingBackgroundColor != aOther.fontSmoothingBackgroundColor)) {
+    return MaxDifference::eVisual;
+  }
+
+  return MaxDifference::eNone;
 }
 
 nsFont& nsFont::operator=(const nsFont& aOther) = default;

gfx/src/nsFont.h

@@ -143,6 +143,14 @@ struct nsFont {
 
   nsFont& operator=(const nsFont& aOther);
 
+  enum class MaxDifference : uint8_t {
+    eNone,
+    eVisual,
+    eLayoutAffecting
+  };
+
+  MaxDifference CalcDifference(const nsFont& aOther) const;
+
   void CopyAlternates(const nsFont& aOther);
 
   // Add featureSettings into style

gfx/thebes/gfxFontFamilyList.h

@@ -446,6 +446,11 @@ operator==(const FontFamilyList& a, const FontFamilyList& b) {
     return a.Equals(b);
 }
 
+inline bool
+operator!=(const FontFamilyList& a, const FontFamilyList& b) {
+    return !a.Equals(b);
+}
+
 } // namespace mozilla
 
 #endif /* GFX_FONT_FAMILY_LIST_H */

layout/style/nsStyleStruct.cpp

@@ -199,18 +199,25 @@ nsStyleFont::CalcDifference(const nsStyleFont& aNewData) const
   MOZ_ASSERT(mAllowZoom == aNewData.mAllowZoom,
              "expected mAllowZoom to be the same on both nsStyleFonts");
   if (mSize != aNewData.mSize ||
-      mFont != aNewData.mFont ||
       mLanguage != aNewData.mLanguage ||
       mExplicitLanguage != aNewData.mExplicitLanguage ||
       mMathVariant != aNewData.mMathVariant ||
       mMathDisplay != aNewData.mMathDisplay ||
       mMinFontSizeRatio != aNewData.mMinFontSizeRatio) {
-    // If only mFont.fontSmoothingBackgroundColor changes, we really only need
-    // a repaint hint rather than a reflow+repaint hint, but it's not worth
-    // worth optimizing.
     return NS_STYLE_HINT_REFLOW;
   }
 
+  switch (mFont.CalcDifference(aNewData.mFont)) {
+    case nsFont::MaxDifference::eLayoutAffecting:
+      return NS_STYLE_HINT_REFLOW;
+
+    case nsFont::MaxDifference::eVisual:
+      return NS_STYLE_HINT_VISUAL;
+
+    case nsFont::MaxDifference::eNone:
+      break;
+  }
+
   // XXX Should any of these cause a non-nsChangeHint_NeutralChange change?
   if (mGenericID != aNewData.mGenericID ||
       mScriptLevel != aNewData.mScriptLevel ||