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

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

Index: third_party/WebKit/Source/platform/scroll/ScrollAnimatorCompositorCoordinator.cpp
diff --git a/third_party/WebKit/Source/platform/scroll/ScrollAnimatorCompositorCoordinator.cpp b/third_party/WebKit/Source/platform/scroll/ScrollAnimatorCompositorCoordinator.cpp
index 142b27e77d62f5646402360ceea8281f46b330ca..8edc1cbd74b89d5c9e85912a550e63e4dbddd4dd 100644
--- a/third_party/WebKit/Source/platform/scroll/ScrollAnimatorCompositorCoordinator.cpp
+++ b/third_party/WebKit/Source/platform/scroll/ScrollAnimatorCompositorCoordinator.cpp
@@ -6,6 +6,7 @@
 
 #include "platform/RuntimeEnabledFeatures.h"
 #include "platform/graphics/GraphicsLayer.h"
+#include "platform/scroll/MainThreadScrollingReason.h"
 #include "platform/scroll/ScrollableArea.h"
 #include "public/platform/Platform.h"
 #include "public/platform/WebCompositorAnimationPlayer.h"
@@ -14,6 +15,11 @@
 
 namespace blink {
 
+WebLayer* toWebLayer(blink::GraphicsLayer* layer)
+{
+    return layer ? layer->platformLayer() : nullptr;
+}
+
 ScrollAnimatorCompositorCoordinator::ScrollAnimatorCompositorCoordinator()
     : m_compositorAnimationAttachedToLayerId(0)
     , m_runState(RunState::Idle)
@@ -41,6 +47,26 @@ void ScrollAnimatorCompositorCoordinator::resetAnimationState()
     m_runState = RunState::Idle;
     m_compositorAnimationId = 0;
     m_compositorAnimationGroupId = 0;
+
+    // Remove the temporary main thread scrolling reason that was added while
+    // main thread had scheduled an animation.
+    removeMainThreadScrollingReason();
+}
+
+void ScrollAnimatorCompositorCoordinator::addMainThreadScrollingReason()
+{
+    if (WebLayer* scrollLayer = toWebLayer(scrollableArea()->layerForScrolling())) {
+        scrollLayer->addMainThreadScrollingReasons(
+            MainThreadScrollingReason::kAnimatingScollOnMainThread);
+    }
+}
+
+void ScrollAnimatorCompositorCoordinator::removeMainThreadScrollingReason()
+{
+    if (WebLayer* scrollLayer = toWebLayer(scrollableArea()->layerForScrolling())) {
+        scrollLayer->clearMainThreadScrollingReasons(
+            MainThreadScrollingReason::kAnimatingScollOnMainThread);
+    }
 }
 
 bool ScrollAnimatorCompositorCoordinator::hasAnimationThatRequiresService() const
@@ -167,6 +193,8 @@ void ScrollAnimatorCompositorCoordinator::reattachCompositorPlayerIfNeeded(
                     scrollableArea()->layerForScrolling()->platformLayer());
             }
             m_compositorAnimationAttachedToLayerId = compositorAnimationAttachedToLayerId;
+            if (hasRunningAnimation())
+                addMainThreadScrollingReason();

[ajuma, 2016/02/04 20:08:40]

Will this also affect input-driven smooth scroll (given that it's in the base
class rather than in ScrollAnimator)?

[ymalik, 2016/02/04 21:53:20]

You're right, this should not affect non*-input driven scroll. Moved the call
the clearing and adding to ScrollAnimator :).

         }
     }
 }