Disable overlay scrollbars in Blink when hidden by the compositor.

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 234 matching lines @@
   // animations.
   virtual bool scheduleAnimation();
   virtual void serviceScrollAnimations(double monotonicTime);
   virtual void updateCompositorScrollAnimations();
   virtual void registerForAnimation() {}
   virtual void deregisterForAnimation() {}
 
   virtual bool usesCompositedScrolling() const { return false; }
   virtual bool shouldScrollOnMainThread() const;
 
+  // Overlay scrollbars can "fade-out" when inactive.
+  virtual bool scrollbarsHidden() const;
+  virtual void setScrollbarsHidden(bool);
+  virtual void didChangeScrollbarsHidden(){};
+
   // Returns true if the GraphicsLayer tree needs to be rebuilt.
   virtual bool updateAfterCompositingChange() { return false; }
 
   virtual bool userInputScrollable(ScrollbarOrientation) const = 0;
   virtual bool shouldPlaceVerticalScrollbarOnLeft() const = 0;
 
   // Convenience functions
   float scrollOffset(ScrollbarOrientation orientation) {
     return orientation == HorizontalScrollbar ? scrollOffsetInt().width()
                                               : scrollOffsetInt().height();
@@ skipping 125 matching lines @@
   mutable Member<ScrollAnimatorBase> m_scrollAnimator;
   mutable Member<ProgrammaticScrollAnimator> m_programmaticScrollAnimator;
 
   unsigned m_scrollbarOverlayStyle : 2;  // ScrollbarOverlayStyle
 
   unsigned m_scrollOriginChanged : 1;
 
   unsigned m_horizontalScrollbarNeedsPaintInvalidation : 1;
   unsigned m_verticalScrollbarNeedsPaintInvalidation : 1;
   unsigned m_scrollCornerNeedsPaintInvalidation : 1;
+  unsigned m_scrollbarsHidden : 1;
 
   // There are 6 possible combinations of writing mode and direction. Scroll
   // origin will be non-zero in the x or y axis if there is any reversed
   // direction or writing-mode. The combinations are:
   // writing-mode / direction     scrollOrigin.x() set    scrollOrigin.y() set
   // horizontal-tb / ltr          NO                      NO
   // horizontal-tb / rtl          YES                     NO
   // 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