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

third_party/WebKit/Source/core/frame/FrameView.h

 /*
    Copyright (C) 1997 Martin Jones (mjones@kde.org)
              (C) 1998 Waldo Bastian (bastian@kde.org)
              (C) 1998, 1999 Torben Weis (weis@kde.org)
              (C) 1999 Lars Knoll (knoll@kde.org)
              (C) 1999 Antti Koivisto (koivisto@kde.org)
    Copyright (C) 2004, 2005, 2006, 2007, 2008, 2009 Apple Inc. 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 739 matching lines @@
     IntRect rectToCopyOnScroll() const;
 
     bool isFrameViewScrollbar(const Widget* child) const { return horizontalScrollbar() == child || verticalScrollbar() == child; }
 
     ScrollingCoordinator* scrollingCoordinator() const;
 
     void prepareLayoutAnalyzer();
     std::unique_ptr<TracedValue> analyzerCounters();
 
     // LayoutObject for the viewport-defining element (see Document::viewportDefiningElement).
-    LayoutObject* viewportLayoutObject();
+    LayoutObject* viewportLayoutObject() const;
 
     void collectAnnotatedRegions(LayoutObject&, Vector<AnnotatedRegionValue>&) const;
 
     template <typename Function> void forAllNonThrottledFrameViews(const Function&);
 
     void setNeedsUpdateViewportIntersection();
     void updateViewportIntersectionsForSubtree(DocumentLifecycle::LifecycleState targetState);
     void updateViewportIntersectionIfNeeded();
     void notifyRenderThrottlingObservers();
     void updateThrottlingStatus();
@@ skipping 178 matching lines @@
     if (m_visuallyNonEmptyPixelCount > visualPixelThreshold)
         setIsVisuallyNonEmpty();
 }
 
 DEFINE_TYPE_CASTS(FrameView, Widget, widget, widget->isFrameView(), widget.isFrameView());
 DEFINE_TYPE_CASTS(FrameView, ScrollableArea, scrollableArea, scrollableArea->isFrameView(), scrollableArea.isFrameView());
 
 } // namespace blink
 
 #endif // FrameView_h