Don't update graphics layer positions during coordinated scrolling

Source/core/page/scrolling/ScrollingCoordinator.cpp

 /*
  * Copyright (C) 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 249 matching lines @@
         bool isOpaqueScrollbar = !scrollbar->isOverlayScrollbar();
         if (!scrollbarGraphicsLayer->contentsOpaque())
             scrollbarGraphicsLayer->setContentsOpaque(isMainFrame && isOpaqueScrollbar);
         scrollbarLayer->layer()->setOpaque(scrollbarGraphicsLayer->contentsOpaque());
 
         setupScrollbarLayer(scrollbarGraphicsLayer, scrollbarLayer, scrollingWebLayerForScrollableArea(scrollableArea));
     } else
         removeWebScrollbarLayer(scrollableArea, orientation);
 }
 
-void ScrollingCoordinator::scrollableAreaScrollLayerDidChange(ScrollableArea* scrollableArea)
+bool ScrollingCoordinator::scrollableAreaScrollLayerDidChange(ScrollableArea* scrollableArea)
 {
     GraphicsLayer* scrollLayer = scrollLayerForScrollableArea(scrollableArea);
     if (scrollLayer) {
         bool isMainFrame = isForMainFrame(scrollableArea);
         scrollLayer->setScrollableArea(scrollableArea, isMainFrame);
     }
 
     WebLayer* webLayer = scrollingWebLayerForScrollableArea(scrollableArea);
     if (webLayer) {
         webLayer->setScrollable(true);
         webLayer->setScrollPosition(IntPoint(scrollableArea->scrollPosition() - scrollableArea->minimumScrollPosition()));
         webLayer->setMaxScrollPosition(IntSize(scrollableArea->scrollSize(HorizontalScrollbar), scrollableArea->scrollSize(VerticalScrollbar)));
     }
     if (WebScrollbarLayer* scrollbarLayer = getWebScrollbarLayer(scrollableArea, HorizontalScrollbar))
         setupScrollbarLayer(horizontalScrollbarLayerForScrollableArea(scrollableArea), scrollbarLayer, webLayer);
     if (WebScrollbarLayer* scrollbarLayer = getWebScrollbarLayer(scrollableArea, VerticalScrollbar))
         setupScrollbarLayer(verticalScrollbarLayerForScrollableArea(scrollableArea), scrollbarLayer, webLayer);
+
+    return !!webLayer;
 }
 
 void ScrollingCoordinator::setTouchEventTargetRects(const Vector<IntRect>& absoluteHitTestRects)
 {
     if (WebLayer* scrollLayer = scrollingWebLayerForScrollableArea(m_page->mainFrame()->view())) {
         WebVector<WebRect> webRects(absoluteHitTestRects.size());
         for (size_t i = 0; i < absoluteHitTestRects.size(); ++i)
             webRects[i] = absoluteHitTestRects[i];
         scrollLayer->setTouchEventHandlerRegion(webRects);
     }
@@ skipping 341 matching lines @@
         stringBuilder.resize(stringBuilder.length() - 2);
     return stringBuilder.toString();
 }
 
 String ScrollingCoordinator::mainThreadScrollingReasonsAsText() const
 {
     return mainThreadScrollingReasonsAsText(mainThreadScrollingReasons());
 }
 
 } // namespace WebCore