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

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

 /*
  * Copyright (C) 2010 Apple Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1.  Redistributions of source code must retain the above copyright
  *     notice, this list of conditions and the following disclaimer.
  * 2.  Redistributions in binary form must reproduce the above copyright
  *     notice, this list of conditions and the following disclaimer in the
@@ skipping 80 matching lines @@
 
     LayoutFlexibleBox::willBeDestroyed();
 }
 
 void LayoutFullScreen::updateStyle()
 {
     RefPtr<ComputedStyle> fullscreenStyle = ComputedStyle::create();
 
     // Create a stacking context:
     fullscreenStyle->setZIndex(INT_MAX);
+    fullscreenStyle->setIsStackingContext(true);
 
     fullscreenStyle->setFontDescription(FontDescription());
     fullscreenStyle->font().update(nullptr);
 
     fullscreenStyle->setDisplay(FLEX);
     fullscreenStyle->setJustifyContentPosition(ContentPositionCenter);
     fullscreenStyle->setAlignItemsPosition(ItemPositionCenter);
     fullscreenStyle->setFlexDirection(FlowColumn);
 
     fullscreenStyle->setPosition(FixedPosition);
@@ skipping 85 matching lines @@
         m_placeholder->setStyleWithWritingModeOfParent(style);
         if (parent()) {
             parent()->addChildWithWritingModeOfParent(m_placeholder, this);
             parent()->setNeedsLayoutAndPrefWidthsRecalcAndFullPaintInvalidation(LayoutInvalidationReason::Fullscreen);
         }
     } else {
         m_placeholder->setStyle(style);
         m_placeholder->setStyleWithWritingModeOfParent(style);
     }
 }