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

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

Index: third_party/WebKit/Source/platform/scroll/ScrollAnimator.cpp
diff --git a/third_party/WebKit/Source/platform/scroll/ScrollAnimator.cpp b/third_party/WebKit/Source/platform/scroll/ScrollAnimator.cpp
index de0000b4a8f5a948cb00f564d08a62014024fb01..e71a9abd110c6a541fcf624ea6ba98ddc303292a 100644
--- a/third_party/WebKit/Source/platform/scroll/ScrollAnimator.cpp
+++ b/third_party/WebKit/Source/platform/scroll/ScrollAnimator.cpp
@@ -32,6 +32,7 @@
 
 #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"
@@ -41,6 +42,11 @@
 
 namespace blink {
 
+WebLayer* toWebLayer(blink::GraphicsLayer* layer)
+{
+    return layer ? layer->platformLayer() : nullptr;
+}
+
 PassOwnPtrWillBeRawPtr<ScrollAnimatorBase> ScrollAnimatorBase::create(ScrollableArea* scrollableArea)
 {
     if (scrollableArea && scrollableArea->scrollAnimatorEnabled())
@@ -85,6 +91,10 @@ void ScrollAnimator::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(
@@ -259,10 +269,33 @@ void ScrollAnimator::updateCompositorAnimations()
             }
         }
 
+        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);
     }
 }
 
@@ -286,7 +319,8 @@ void ScrollAnimator::cancelAnimation()
 void ScrollAnimator::layerForCompositedScrollingDidChange(
     WebCompositorAnimationTimeline* timeline)
 {
-    reattachCompositorPlayerIfNeeded(timeline);
+    if (reattachCompositorPlayerIfNeeded(timeline) && m_animationCurve)
+        addMainThreadScrollingReason();
 }
 
 bool ScrollAnimator::registerAndScheduleAnimation()