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

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

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "platform/scroll/ScrollAnimatorCompositorCoordinator.h"
 
 #include "platform/RuntimeEnabledFeatures.h"
 #include "platform/graphics/GraphicsLayer.h"
 #include "platform/scroll/ScrollableArea.h"
 #include "public/platform/Platform.h"
@@ skipping 132 matching lines @@
     case RunState::RunningOnCompositor:
     case RunState::RunningOnCompositorButNeedsUpdate:
     case RunState::WaitingToCancelOnCompositor:
         m_runState = RunState::PostAnimationCleanup;
 
         // Get serviced the next time compositor updates are allowed.
         scrollableArea()->registerForAnimation();
     }
 }
 
-void ScrollAnimatorCompositorCoordinator::reattachCompositorPlayerIfNeeded(
+bool ScrollAnimatorCompositorCoordinator::reattachCompositorPlayerIfNeeded(
     WebCompositorAnimationTimeline* timeline)
 {
+    bool reattached = false;
     int compositorAnimationAttachedToLayerId = 0;
     if (scrollableArea()->layerForScrolling())
         compositorAnimationAttachedToLayerId = scrollableArea()->layerForScrolling()->platformLayer()->id();
 
     if (compositorAnimationAttachedToLayerId != m_compositorAnimationAttachedToLayerId) {
         if (m_compositorPlayer && timeline) {
             // Detach from old layer (if any).
             if (m_compositorAnimationAttachedToLayerId) {
                 if (m_compositorPlayer->isLayerAttached())
                     m_compositorPlayer->detachLayer();
                 timeline->playerDestroyed(*this);
             }
             // Attach to new layer (if any).
             if (compositorAnimationAttachedToLayerId) {
                 ASSERT(!m_compositorPlayer->isLayerAttached());
                 timeline->playerAttached(*this);
                 m_compositorPlayer->attachLayer(
                     scrollableArea()->layerForScrolling()->platformLayer());
+                reattached = true;
             }
             m_compositorAnimationAttachedToLayerId = compositorAnimationAttachedToLayerId;
         }
     }
+
+    return reattached;
 }
 
 void ScrollAnimatorCompositorCoordinator::notifyAnimationStarted(
     double monotonicTime, int group)
 {
 }
 
 void ScrollAnimatorCompositorCoordinator::notifyAnimationFinished(
     double monotonicTime, int group)
 {
     notifyCompositorAnimationFinished(group);
 }
 
 void ScrollAnimatorCompositorCoordinator::notifyAnimationAborted(
     double monotonicTime, int group)
 {
     // An animation aborted by the compositor is treated as a finished
     // animation.
     notifyCompositorAnimationFinished(group);
 }
 
 WebCompositorAnimationPlayer* ScrollAnimatorCompositorCoordinator::compositorPlayer() const
 {
     return m_compositorPlayer.get();
 }
 
 } // namespace blink