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..abe02308ae41bdc775047555b70b1fdd13fe96ff 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"
@@ -14,11 +15,17 @@
 
 namespace blink {
 
+WebLayer* toWebLayer(blink::GraphicsLayer* layer)
+{
+    return layer ? layer->platformLayer() : nullptr;
+}
+
 ScrollAnimatorCompositorCoordinator::ScrollAnimatorCompositorCoordinator()
     : m_compositorAnimationAttachedToLayerId(0)
     , m_runState(RunState::Idle)
     , m_compositorAnimationId(0)
     , m_compositorAnimationGroupId(0)
+    , m_clearMainThreadScrollingReasons(false)
 {
     if (RuntimeEnabledFeatures::compositorAnimationTimelinesEnabled()) {
         ASSERT(Platform::current()->compositorSupport());
@@ -41,6 +48,42 @@ void ScrollAnimatorCompositorCoordinator::resetAnimationState()
     m_runState = RunState::Idle;
     m_compositorAnimationId = 0;
     m_compositorAnimationGroupId = 0;
+
+    // While there was a running animation, a temporary main thread scrolling
+    // reason may have been added for the animation to complete on the main
+    // thread.
+    clearMainThreadScrollingReasonsOnLayer();
+}
+
+void ScrollAnimatorCompositorCoordinator::clearMainThreadScrollingReasonsOnLayer()
+{
+    if (WebLayer* scrollLayer = toWebLayer(scrollableArea()->layerForScrolling())) {
+        if (m_clearMainThreadScrollingReasons) {
+            scrollLayer->clearMainThreadScrollingReasons();
+            m_clearMainThreadScrollingReasons = false;
+        } else {
+            scrollLayer->clearMainThreadScrollingReasons(
+                MainThreadScrollingReason::kContinuingMainThreadScroll);
+        }
+    }
+}
+
+void ScrollAnimatorCompositorCoordinator::clearMainThreadScrollingReasons(bool clear)
+{
+    m_clearMainThreadScrollingReasons = clear;
+    if (!clear)
+        return;
+    if (!hasRunningAnimation()) {
+        clearMainThreadScrollingReasonsOnLayer();
+    } else if (WebLayer* scrollLayer = toWebLayer(scrollableArea()->layerForScrolling())) {
+        // If there is a running animation, finish it on main thread before
+        // clearing main thread scrolling reasons. In the mean time, add
+        // a temporary main thread scrolling reason that ensures that the cc
+        // doesn't start another scroll animation that will compete with this
+        // animation (see crbug.com/581875).
+        scrollLayer->addMainThreadScrollingReasons(
+            MainThreadScrollingReason::kContinuingMainThreadScroll);
+    }
 }
 
 bool ScrollAnimatorCompositorCoordinator::hasAnimationThatRequiresService() const