Fix smooth scroll jump when switching scroll handling between MT and CC

third_party/WebKit/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 326 matching lines @@
     static const bool platformSupportsCoordinatedScrollbar = true;
 #endif
 
     bool isMainFrame = isForMainFrame(scrollableArea);
     GraphicsLayer* scrollbarGraphicsLayer = orientation == HorizontalScrollbar
         ? scrollableArea->layerForHorizontalScrollbar()
         : scrollableArea->layerForVerticalScrollbar();
 
     if (!platformSupportsCoordinatedScrollbar) {
         if (scrollbarGraphicsLayer) {
-            WebLayer* scrollbarLayer = toWebLayer(scrollbarGraphicsLayer);
-            scrollbarLayer->addMainThreadScrollingReasons(MainThreadScrollingReason::kScrollbarScrolling);
+            scrollbarGraphicsLayer->setMainThreadScrollingReasons(MainThreadScrollingReason::kScrollbarScrolling);

[ajuma, 2016/02/04 15:05:48]

Is changing this from adding a reason to replacing the existing reasons
intentional?

         }
         return;
     }
 
     if (scrollbarGraphicsLayer) {
         Scrollbar& scrollbar = orientation == HorizontalScrollbar ? *scrollableArea->horizontalScrollbar() : *scrollableArea->verticalScrollbar();
         if (scrollbar.isCustomScrollbar()) {
             detachScrollbarLayer(scrollbarGraphicsLayer);
             return;
         }
@@ skipping 318 matching lines @@
 
 void ScrollingCoordinator::willDestroyLayer(PaintLayer* layer)
 {
     m_layersWithTouchRects.remove(layer);
 }
 
 void ScrollingCoordinator::setShouldUpdateScrollLayerPositionOnMainThread(MainThreadScrollingReasons mainThreadScrollingReasons)
 {
     if (!m_page->mainFrame()->isLocalFrame() || !m_page->deprecatedLocalMainFrame()->view())
         return;
-    if (WebLayer* scrollLayer = toWebLayer(m_page->deprecatedLocalMainFrame()->view()->layerForScrolling())) {
+    if (GraphicsLayer* scrollLayer = m_page->deprecatedLocalMainFrame()->view()->layerForScrolling()) {
         m_lastMainThreadScrollingReasons = mainThreadScrollingReasons;
-        if (mainThreadScrollingReasons)
-            scrollLayer->addMainThreadScrollingReasons(mainThreadScrollingReasons);
-        else
-            scrollLayer->clearMainThreadScrollingReasons();
+        scrollLayer->setMainThreadScrollingReasons(mainThreadScrollingReasons);
     }
 }
 
 void ScrollingCoordinator::layerTreeViewInitialized(WebLayerTreeView& layerTreeView)
 {
     if (RuntimeEnabledFeatures::compositorAnimationTimelinesEnabled() && Platform::current()->isThreadedAnimationEnabled()) {
         ASSERT(Platform::current()->compositorSupport());
         m_programmaticScrollAnimatorTimeline = adoptPtr(Platform::current()->compositorSupport()->createAnimationTimeline());
         layerTreeView.attachCompositorAnimationTimeline(m_programmaticScrollAnimatorTimeline.get());
     }
@@ skipping 348 matching lines @@
     bool frameIsScrollable = frameView && frameView->isScrollable();
     if (frameIsScrollable != m_wasFrameScrollable)
         return true;
 
     if (WebLayer* scrollLayer = frameView ? toWebLayer(frameView->layerForScrolling()) : nullptr)
         return WebSize(frameView->contentsSize()) != scrollLayer->bounds();
     return false;
 }
 
 } // namespace blink