Changing from opacity less than 1 to opacity 1 should not trigger layout

Source/core/rendering/style/RenderStyle.cpp

Index: Source/core/rendering/style/RenderStyle.cpp
diff --git a/Source/core/rendering/style/RenderStyle.cpp b/Source/core/rendering/style/RenderStyle.cpp
index 64bd27767603576930aa858ea699211cb535c4ee..8d1f8dadef7fba33d45c24e845fbea67acd9682f 100644
--- a/Source/core/rendering/style/RenderStyle.cpp
+++ b/Source/core/rendering/style/RenderStyle.cpp
@@ -570,7 +570,9 @@ StyleDifference RenderStyle::diff(const RenderStyle* other, unsigned& changedCon
     if ((visibility() == COLLAPSE) != (other->visibility() == COLLAPSE))
         return StyleDifferenceLayout;
 
-    if (rareNonInheritedData->hasOpacity() != other->rareNonInheritedData->hasOpacity()) {
+    // We only need do layout for opacity changes if adding or losing opacity could trigger a change
+    // in us being a stacking context.
+    if (hasAutoZIndex() != other->hasAutoZIndex() && rareNonInheritedData->hasOpacity() != other->rareNonInheritedData->hasOpacity()) {
         // FIXME: We would like to use SimplifiedLayout here, but we can't quite do that yet.
         // We need to make sure SimplifiedLayout can operate correctly on RenderInlines (we will need
         // to add a selfNeedsSimplifiedLayout bit in order to not get confused and taint every line).