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

third_party/WebKit/Source/core/style/ComputedStyle.h

 /*
  * Copyright (C) 2000 Lars Knoll (knoll@kde.org)
  *           (C) 2000 Antti Koivisto (koivisto@kde.org)
  *           (C) 2000 Dirk Mueller (mueller@kde.org)
  * Copyright (C) 2003, 2005, 2006, 2007, 2008, 2009, 2010, 2011 Apple Inc. All rights reserved.
  * Copyright (C) 2006 Graham Dennis (graham.dennis@gmail.com)
  *
  * 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
@@ skipping 1257 matching lines @@
     void clearCursorList();
 
     void setInsideLink(EInsideLink insideLink) { inherited_flags._insideLink = insideLink; }
     void setIsLink(bool b) { noninherited_flags.isLink = b; }
 
     PrintColorAdjust getPrintColorAdjust() const { return static_cast<PrintColorAdjust>(inherited_flags.m_printColorAdjust); }
     void setPrintColorAdjust(PrintColorAdjust value) { inherited_flags.m_printColorAdjust = value; }
 
     // A stacking context is painted atomically and defines a stacking order, whereas
     // a containing stacking context defines in which order the stacking contexts
-    // below are painted. In Blink, a stacking context is defined by non-auto
-    // z-index'. This invariant is enforced by the logic in StyleAdjuster
+    // below are painted.
     // See CSS 2.1, Appendix E (https://www.w3.org/TR/CSS21/zindex.html) for more details.
-    bool isStackingContext() const { return !hasAutoZIndex(); }
+    bool isStackingContext() const { return rareNonInheritedData->m_isStackingContext; }
+
+    void updateIsStackingContext(bool isDocumentElement, bool isInTopLayer);
+    void setIsStackingContext(bool b) { SET_VAR(rareNonInheritedData, m_isStackingContext, b); }
 
     // Stacking contexts and positioned elements[1] are stacked (sorted in negZOrderList
     // and posZOrderList) in their enclosing stacking contexts.
     //
     // [1] According to CSS2.1, Appendix E.2.8 (https://www.w3.org/TR/CSS21/zindex.html),
     // positioned elements with 'z-index: auto' are "treated as if it created a new
     // stacking context" and z-ordered together with other elements with 'z-index: 0'.
     // The difference of them from normal stacking contexts is that they don't determine
     // the stacking of the elements underneath them.
     // (Note: There are also other elements treated as stacking context during painting,
@@ skipping 733 matching lines @@
 }
 
 inline bool ComputedStyle::hasPseudoElementStyle() const
 {
     return noninherited_flags.pseudoBits & ElementPseudoIdMask;
 }
 
 } // namespace blink
 
 #endif // ComputedStyle_h