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 98f4e1350a715ff5aeb0842ba07932d822046528..660125e66a8c8e1cd99cf785df6def1830ab3604 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)

[jbroman, 2016/02/10 20:36:37]

Please make this a static function or enclose it in an anonymous namespace. It's
not advertised in the header, and it could conceivably collide with another
function sharing this name.

Also, this is in the blink namespace, so please remove the blink:: namespace
qualification.

[ymalik, 2016/02/10 21:57:58]

Done. Thanks!

+{
+    return layer ? layer->platformLayer() : nullptr;
+}
+
 PassOwnPtrWillBeRawPtr<ScrollAnimatorBase> ScrollAnimatorBase::create(ScrollableArea* scrollableArea)
 {
     if (scrollableArea && scrollableArea->scrollAnimatorEnabled())
@@ -189,10 +195,21 @@ void ScrollAnimator::tickAnimation(double monotonicTime)
     notifyPositionChanged();
 }
 
+void ScrollAnimator::postAnimationCleanupAndReset()
+{
+    // Remove the temporary main thread scrolling reason that was added while
+    // main thread had scheduled an animation.
+    removeMainThreadScrollingReason();
+
+    resetAnimationState();
+}
+
 void ScrollAnimator::updateCompositorAnimations()
 {
-    if (m_runState == RunState::PostAnimationCleanup)
-        return resetAnimationState();
+    if (m_runState == RunState::PostAnimationCleanup) {
+        postAnimationCleanupAndReset();
+        return;
+    }
 
     if (m_compositorAnimationId && m_runState != RunState::RunningOnCompositor
         && m_runState != RunState::RunningOnCompositorButNeedsUpdate) {
@@ -208,7 +225,8 @@ void ScrollAnimator::updateCompositorAnimations()
         m_compositorAnimationId = 0;
         m_compositorAnimationGroupId = 0;
         if (m_runState == RunState::WaitingToCancelOnCompositor) {
-            return resetAnimationState();
+            postAnimationCleanupAndReset();
+            return;
         }
     }
 
@@ -263,10 +281,34 @@ 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();
+        else
+            removeMainThreadScrollingReason();
+    }
+}
+
+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);
     }
 }
 
@@ -290,7 +332,8 @@ void ScrollAnimator::cancelAnimation()
 void ScrollAnimator::layerForCompositedScrollingDidChange(
     WebCompositorAnimationTimeline* timeline)
 {
-    reattachCompositorPlayerIfNeeded(timeline);
+    if (reattachCompositorPlayerIfNeeded(timeline) && m_animationCurve)
+        addMainThreadScrollingReason();
 }
 
 bool ScrollAnimator::registerAndScheduleAnimation()