Correctly reset ScrollAnimator animation state after cancelling animation

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 ab02ae1b29876a54f6bf9a60375de1bec3c4b0fc..af2340832e8dad8845e7a8f6ababeabbfe2ac122 100644
--- a/third_party/WebKit/Source/platform/scroll/ScrollAnimator.cpp
+++ b/third_party/WebKit/Source/platform/scroll/ScrollAnimator.cpp
@@ -112,6 +112,10 @@ ScrollResult ScrollAnimator::userScroll(
         return ScrollAnimatorBase::userScroll(granularity, delta);
     }
 
+    bool needsPostAnimationCleanup = m_runState == RunState::PostAnimationCleanup;
+    if (m_runState == RunState::PostAnimationCleanup)
+        resetAnimationState();
+
     FloatSize consumedDelta = computeDeltaToConsume(delta);
 
     FloatPoint targetPos = desiredTargetPosition();
@@ -125,6 +129,13 @@ ScrollResult ScrollAnimator::userScroll(
         // unusedDelta. This differs from ScrollAnimatorMac currently.
         return ScrollResult(true, true, 0, 0);
     }
+
+    // If the run state when this method was called was PostAnimationCleanup and
+    // we're not starting an animation, stay in PostAnimationCleanup state so
+    // that the main thread scrolling reason can be removed.
+    if (needsPostAnimationCleanup)
+        m_runState = RunState::PostAnimationCleanup;
+
     // Report unused delta only if there is no animation and we are not
     // starting one. This ensures we latch for the duration of the
     // animation rather than animating multiple scrollers at the same time.