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

Source/core/rendering/style/RenderStyle.cpp

 /*
  * Copyright (C) 1999 Antti Koivisto (koivisto@kde.org)
  * Copyright (C) 2004, 2005, 2006, 2007, 2008, 2009, 2010 Apple Inc. All rights reserved.
  * Copyright (C) 2011 Adobe Systems Incorporated. All rights reserved.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
  * License as published by the Free Software Foundation; either
  * version 2 of the License, or (at your option) any later version.
  *
@@ skipping 552 matching lines @@
 
     // If the counter directives change, trigger a relayout to re-calculate counter values and rebuild the counter node tree.
     const CounterDirectiveMap* mapA = rareNonInheritedData->m_counterDirectives.get();
     const CounterDirectiveMap* mapB = other->rareNonInheritedData->m_counterDirectives.get();
     if (!(mapA == mapB || (mapA && mapB && *mapA == *mapB)))
         return StyleDifferenceLayout;
 
     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).
         // In addition we need to solve the floating object issue when layers come and go. Right now
         // a full layout is necessary to keep floating object lists sane.
         return StyleDifferenceLayout;
     }
 
     if (rareNonInheritedData->hasFilters() != other->rareNonInheritedData->hasFilters())
         return StyleDifferenceLayout;
@@ skipping 1018 matching lines @@
 }
 
 void RenderStyle::setBorderImageOutset(const BorderImageLengthBox& outset)
 {
     if (surround->border.m_image.outset() == outset)
         return;
     surround.access()->border.m_image.setOutset(outset);
 }
 
 } // namespace WebCore