Avoid touching z-index in StyleAdjuster by using an isStackingContext flag instead

third_party/WebKit/Source/core/layout/LayoutBox.cpp

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2005 Allan Sandfeld Jensen (kde@carewolf.com)
  *           (C) 2005, 2006 Samuel Weinig (sam.weinig@gmail.com)
  * Copyright (C) 2005, 2006, 2007, 2008, 2009, 2010 Apple Inc. All rights reserved.
  * Copyright (C) 2013 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
@@ skipping 90 matching lines @@
 {
     setIsBox();
 }
 
 PaintLayerType LayoutBox::layerTypeRequired() const
 {
     // hasAutoZIndex only returns true if the element is positioned or a flex-item since
     // position:static elements that are not flex-items get their z-index coerced to auto.
     if (isPositioned() || createsGroup() || hasClipPath() || hasTransformRelatedProperty()
         || style()->hasCompositorProxy() || hasHiddenBackface() || hasReflection() || style()->specifiesColumns()
-        || !style()->hasAutoZIndex() || style()->shouldCompositeForCurrentAnimations())
+        || style()->isStackingContext() || style()->shouldCompositeForCurrentAnimations())
         return NormalPaintLayer;
 
     if (hasOverflowClip())
         return OverflowClipPaintLayer;
 
     return NoPaintLayer;
 }
 
 void LayoutBox::willBeDestroyed()
 {
@@ skipping 1259 matching lines @@
         return false;
     if (childStyle.visibility() != VISIBLE || childStyle.shapeOutside())
         return false;
     if (childBox.size().isZero())
         return false;
     if (PaintLayer* childLayer = childBox.layer()) {
         // FIXME: perhaps this could be less conservative?
         if (childLayer->compositingState() != NotComposited)
             return false;
         // FIXME: Deal with z-index.
-        if (!childStyle.hasAutoZIndex())
+        if (childStyle.isStackingContext())
             return false;
         if (childLayer->hasTransformRelatedProperty() || childLayer->isTransparent() || childLayer->hasFilterInducingProperty())
             return false;
         if (childBox.hasOverflowClip() && childStyle.hasBorderRadius())
             return false;
     }
     return true;
 }
 
 bool LayoutBox::foregroundIsKnownToBeOpaqueInRect(const LayoutRect& localRect, unsigned maxDepthToTest) const
@@ skipping 3475 matching lines @@
         m_rareData->m_snapAreas->remove(&snapArea);
     }
 }
 
 SnapAreaSet* LayoutBox::snapAreas() const
 {
     return m_rareData ? m_rareData->m_snapAreas.get() : nullptr;
 }
 
 } // namespace blink