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

third_party/WebKit/Source/platform/scroll/ScrollableArea.cpp

 /*
  * Copyright (c) 2010, Google Inc. All rights reserved.
  * 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:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
@@ skipping 19 matching lines @@
  */
 
 #include "platform/scroll/ScrollableArea.h"
 
 #include "platform/HostWindow.h"
 #include "platform/Logging.h"
 #include "platform/geometry/DoubleRect.h"
 #include "platform/geometry/FloatPoint.h"
 #include "platform/geometry/LayoutRect.h"
 #include "platform/graphics/GraphicsLayer.h"
+#include "platform/scroll/MainThreadScrollingReason.h"
 #include "platform/scroll/ProgrammaticScrollAnimator.h"
 #include "platform/scroll/ScrollbarTheme.h"
 #include "wtf/PassOwnPtr.h"
 
 #include "platform/TraceEvent.h"
 
 static const int kPixelsPerLineStep = 40;
 static const float kMinFractionToStepWhenPaging = 0.875f;
 
 namespace blink {
@@ skipping 473 matching lines @@
 
 void ScrollableArea::cancelProgrammaticScrollAnimation()
 {
     if (ProgrammaticScrollAnimator* programmaticScrollAnimator = existingProgrammaticScrollAnimator())
         programmaticScrollAnimator->cancelAnimation();
 }
 
 bool ScrollableArea::shouldScrollOnMainThread() const
 {
     if (GraphicsLayer* layer = layerForScrolling()) {
-        return layer->platformLayer()->shouldScrollOnMainThread();
+        uint32_t reasons = layer->platformLayer()->mainThreadScrollingReasons();
+        // Should scroll on main thread unless the reason is the one that is set
+        // by the ScrollAnimator, in which case, the animation can still be
+        // scheduled on the compositor.
+        // TODO(ymalik): We have a non-transient "main thread scrolling reason"
+        // that doesn't actually cause shouldScrollOnMainThread() to be true.
+        // This is confusing and should be cleaned up.
+        return !!(reasons & ~MainThreadScrollingReason::kAnimatingScrollOnMainThread);
     }
     return true;
 }
 
 DoubleRect ScrollableArea::visibleContentRectDouble(IncludeScrollbarsInRect scrollbarInclusion) const
 {
     return visibleContentRect(scrollbarInclusion);
 }
 
 IntRect ScrollableArea::visibleContentRect(IncludeScrollbarsInRect scrollbarInclusion) const
@@ skipping 65 matching lines @@
         std::max(0, size.height() - horizontalScrollbarHeight));
 }
 
 DEFINE_TRACE(ScrollableArea)
 {
     visitor->trace(m_scrollAnimator);
     visitor->trace(m_programmaticScrollAnimator);
 }
 
 } // namespace blink