Implement the overflow-anchor CSS property as an opt-out for ScrollAnchoring

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

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2007 David Smith (catfish.man@gmail.com)
  * Copyright (C) 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2011 Apple Inc. All rights reserved.
  * Copyright (C) Research In Motion Limited 2010. 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
@@ skipping 346 matching lines @@
     // Update our scroll information if we're overflow:auto/scroll/hidden now that we know if
     // we overflow or not.
     if (hasOverflowClip())
         layer()->getScrollableArea()->updateAfterLayout();
 }
 
 void LayoutBlock::layout()
 {
     LayoutAnalyzer::Scope analyzer(*this);
 
-    bool needsScrollAnchoring = RuntimeEnabledFeatures::scrollAnchoringEnabled() && hasOverflowClip();
+    bool needsScrollAnchoring = hasOverflowClip()
+        && getScrollableArea()->shouldPerformScrollAnchoring();
     if (needsScrollAnchoring)
         getScrollableArea()->scrollAnchor().save();
 
     // Table cells call layoutBlock directly, so don't add any logic here.  Put code into
     // layoutBlock().
     layoutBlock(false);
 
     // It's safe to check for control clip here, since controls can never be table cells.
     // If we have a lightweight clip, there can never be any overflow from children.
     if (hasControlClip() && m_overflow)
@@ skipping 1488 matching lines @@
         for (TrackedLayoutBoxListHashSet::const_iterator it = positionedDescendantSet->begin(); it != end; ++it) {
             LayoutBox* currBox = *it;
             ASSERT(!currBox->needsLayout());
         }
     }
 }
 
 #endif
 
 } // namespace blink