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

third_party/WebKit/Source/platform/graphics/GraphicsLayer.cpp

 /*
  * Copyright (C) 2009 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 1156 matching lines @@
     linkHighlight->layer()->setLayerClient(this);
     updateChildList();
 }
 
 void GraphicsLayer::removeLinkHighlight(LinkHighlight* linkHighlight)
 {
     m_linkHighlights.remove(m_linkHighlights.find(linkHighlight));
     updateChildList();
 }
 
+void GraphicsLayer::setMainThreadScrollingReasons(uint32_t reasons)
+{
+    if (!reasons && m_scrollableArea) {
+        // There may be a scroll animation running on the main thread. Forward
+        // the call to the ScrollAnimator that will clear the main thread
+        // scrolling reasons after the animation is complete.
+        m_scrollableArea->clearMainThreadScrollingReasons(true);
+        return;
+    }
+    if (WebLayer* scrollLayer = platformLayer()) {
+        // The ScrollAnimator will override this main thread scrolling reason if
+        // it is added while the ScrollAnimator is scheduled to clear the main
+        // thread scrolling reason (from call above). Thus, inform it to not
+        // clear the main thread scrolling reasons.
+        m_scrollableArea->clearMainThreadScrollingReasons(false);
+        scrollLayer->addMainThreadScrollingReasons(reasons);

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

Would it simplify things if the main thread scroll animator always adds a main
thread scrolling reason when it starts an animation (or when a composited layer
is created during an animation;
ScrollAnimator::layerForCompositedScrollingDidChange gets called in that case)
and removes this reason when it finishes? Then other code is free to add/remove
other reasons independently, without having to tell the scrollable area.

+    }
+}
+
 void GraphicsLayer::setScrollableArea(ScrollableArea* scrollableArea, bool isViewport)
 {
     if (m_scrollableArea == scrollableArea)
         return;
 
     m_scrollableArea = scrollableArea;
 
     // Viewport scrolling may involve pinch zoom and gets routed through
     // WebViewImpl explicitly rather than via GraphicsLayer::didScroll.
     if (isViewport)
@@ skipping 65 matching lines @@
 {
     if (!layer) {
         fprintf(stderr, "Cannot showGraphicsLayerTree for (nil).\n");
         return;
     }
 
     String output = layer->layerTreeAsText(blink::LayerTreeIncludesDebugInfo);
     fprintf(stderr, "%s\n", output.utf8().data());
 }
 #endif