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

third_party/WebKit/Source/platform/scroll/ScrollableArea.h

 /*
  * Copyright (C) 2008, 2011 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 260 matching lines @@
 
     // Returns the widget associated with this ScrollableArea.
     virtual Widget* getWidget() { return nullptr; }
 
     virtual LayoutBox* layoutBox() const { return nullptr; }
 
     virtual bool isFrameView() const { return false; }
     virtual bool isPaintLayerScrollableArea() const { return false; }
     virtual bool isRootFrameViewport() const { return false; }
 
+    // Returns true if the scroller adjusts the scroll position to compensate
+    // for layout movements (bit.ly/scroll-anchoring).
+    virtual bool shouldPerformScrollAnchoring() const { return false; }
+
     // Need to promptly let go of owned animator objects.
     EAGERLY_FINALIZE();
     DECLARE_VIRTUAL_TRACE();
 
     virtual void clearScrollAnimators();
 
 protected:
     ScrollableArea();
 
     ScrollbarOrientation scrollbarOrientationFromDirection(ScrollDirectionPhysical) const;
@@ skipping 47 matching lines @@
     // vertical-lr / ltr            NO                      NO
     // vertical-lr / rtl            NO                      YES
     // vertical-rl / ltr            YES                     NO
     // vertical-rl / rtl            YES                     YES
     IntPoint m_scrollOrigin;
 };
 
 } // namespace blink
 
 #endif // ScrollableArea_h