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

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

 /*
  * Copyright (c) 2011, Google 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.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 14 matching lines @@
  * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
  * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
  * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
  * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
 
 #include "platform/scroll/ScrollAnimator.h"
 
 #include "platform/TraceEvent.h"
 #include "platform/graphics/GraphicsLayer.h"
+#include "platform/scroll/MainThreadScrollingReason.h"
 #include "platform/scroll/ScrollableArea.h"
 #include "public/platform/Platform.h"
 #include "public/platform/WebCompositorAnimation.h"
 #include "public/platform/WebCompositorSupport.h"
 #include "wtf/CurrentTime.h"
 #include "wtf/PassRefPtr.h"
 
 namespace blink {
 
+WebLayer* toWebLayer(blink::GraphicsLayer* layer)
+{
+    return layer ? layer->platformLayer() : nullptr;
+}
+
 PassOwnPtrWillBeRawPtr<ScrollAnimatorBase> ScrollAnimatorBase::create(ScrollableArea* scrollableArea)
 {
     if (scrollableArea && scrollableArea->scrollAnimatorEnabled())
         return adoptPtrWillBeNoop(new ScrollAnimator(scrollableArea));
     return adoptPtrWillBeNoop(new ScrollAnimatorBase(scrollableArea));
 }
 
 ScrollAnimator::ScrollAnimator(ScrollableArea* scrollableArea, WTF::TimeFunction timeFunction)
     : ScrollAnimatorBase(scrollableArea)
     , m_timeFunction(timeFunction)
@@ skipping 24 matching lines @@
     float newPos = clampScrollPosition(orientation, currentPos + pixelDelta);
     return (currentPos == newPos) ? 0.0f : (newPos - currentPos);
 }
 
 void ScrollAnimator::resetAnimationState()
 {
     ScrollAnimatorCompositorCoordinator::resetAnimationState();
     if (m_animationCurve)
         m_animationCurve.clear();
     m_startTime = 0.0;
+
+    // Remove the temporary main thread scrolling reason that was added while
+    // main thread had scheduled an animation.
+    removeMainThreadScrollingReason();
 }
 
 ScrollResultOneDimensional ScrollAnimator::userScroll(
     ScrollbarOrientation orientation, ScrollGranularity granularity, float step, float delta)
 {
     if (!m_scrollableArea->scrollAnimatorEnabled())
         return ScrollAnimatorBase::userScroll(orientation, granularity, step, delta);
 
     TRACE_EVENT0("blink", "ScrollAnimator::scroll");
 
@@ skipping 154 matching lines @@
             int animationGroupId = animation->group();
 
             sentToCompositor = addAnimation(animation.release());
             if (sentToCompositor) {
                 m_runState = RunState::RunningOnCompositor;
                 m_compositorAnimationId = animationId;
                 m_compositorAnimationGroupId = animationGroupId;
             }
         }
 
+        bool runningOnMainThread = false;
         if (!sentToCompositor) {
-            if (registerAndScheduleAnimation())
+            runningOnMainThread = registerAndScheduleAnimation();
+            if (runningOnMainThread)
                 m_runState = RunState::RunningOnMainThread;
         }
+
+
+        // Main thread should deal with the scroll animations it started.
+        if (sentToCompositor || runningOnMainThread)
+            addMainThreadScrollingReason();
     }
 }
 
+void ScrollAnimator::addMainThreadScrollingReason()
+{
+    if (WebLayer* scrollLayer = toWebLayer(scrollableArea()->layerForScrolling())) {
+        scrollLayer->addMainThreadScrollingReasons(
+            MainThreadScrollingReason::kAnimatingScollOnMainThread);
+    }
+}
+
+void ScrollAnimator::removeMainThreadScrollingReason()
+{
+    if (WebLayer* scrollLayer = toWebLayer(scrollableArea()->layerForScrolling())) {
+        scrollLayer->clearMainThreadScrollingReasons(
+            MainThreadScrollingReason::kAnimatingScollOnMainThread);
+    }
+}
+
 void ScrollAnimator::notifyCompositorAnimationAborted(int groupId)
 {
     // An animation aborted by the compositor is treated as a finished
     // animation.
     ScrollAnimatorCompositorCoordinator::compositorAnimationFinished(groupId);
 }
 
 void ScrollAnimator::notifyCompositorAnimationFinished(int groupId)
 {
     ScrollAnimatorCompositorCoordinator::compositorAnimationFinished(groupId);
 }
 
 void ScrollAnimator::cancelAnimation()
 {
     ScrollAnimatorCompositorCoordinator::cancelAnimation();
 }
 
 void ScrollAnimator::layerForCompositedScrollingDidChange(
     WebCompositorAnimationTimeline* timeline)
 {
-    reattachCompositorPlayerIfNeeded(timeline);
+    if (reattachCompositorPlayerIfNeeded(timeline) && m_animationCurve)
+        addMainThreadScrollingReason();
 }
 
 bool ScrollAnimator::registerAndScheduleAnimation()
 {
     scrollableArea()->registerForAnimation();
     if (!m_scrollableArea->scheduleAnimation()) {
         scrollToOffsetWithoutAnimation(m_targetOffset);
         resetAnimationState();
         return false;
     }
     return true;
 }
 
 DEFINE_TRACE(ScrollAnimator)
 {
     ScrollAnimatorBase::trace(visitor);
 }
 
 } // namespace blink