Don't skip a frame when running smooth scroll animation on the main thread

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 ef7a5d0f7b7b061d3385d9ac59b56a34564fd148..1f24fca4ce78ee500c163adfd50963bd42e307fe 100644
--- a/third_party/WebKit/Source/platform/scroll/ScrollAnimator.cpp
+++ b/third_party/WebKit/Source/platform/scroll/ScrollAnimator.cpp
@@ -186,8 +186,14 @@ bool ScrollAnimator::willAnimateToOffset(const FloatPoint& targetPos)
     m_targetOffset = targetPos;
     m_startTime = m_timeFunction();
 
-    if (registerAndScheduleAnimation())
-        m_runState = RunState::WaitingToSendToCompositor;
+    if (registerAndScheduleAnimation()) {
+        if (m_scrollableArea->shouldScrollOnMainThread()) {
+            createAnimationCurve();
+            m_runState = RunState::RunningOnMainThread;
+        } else {
+            m_runState = RunState::WaitingToSendToCompositor;
+        }
+    }
 
     return true;
 }
@@ -287,9 +293,28 @@ bool ScrollAnimator::sendAnimationToCompositor()
     return sentToCompositor;
 }
 
+void ScrollAnimator::createAnimationCurve()
+{
+    DCHECK(!m_animationCurve);
+    m_animationCurve = adoptPtr(CompositorFactory::current().createScrollOffsetAnimationCurve(
+        compositorOffsetFromBlinkOffset(m_targetOffset),
+        m_lastGranularity == ScrollByPixel ?
+            CompositorScrollOffsetAnimationCurve::ScrollDurationInverseDelta :
+            CompositorScrollOffsetAnimationCurve::ScrollDurationConstant));
+    m_animationCurve->setInitialValue(compositorOffsetFromBlinkOffset(currentPosition()));
+}
+
 void ScrollAnimator::updateCompositorAnimations()
 {
     ScrollAnimatorCompositorCoordinator::updateCompositorAnimations();
+    if (m_runState == RunState::RunningOnMainThread) {
+        // We add a temporary main thread scrolling reason so that subsequent
+        // scrolls get handled on the main thread. This is removed when the
+        // animation is finished in ::tickAnimation.
+        addMainThreadScrollingReason();
+        return;
+    }
+
     if (m_runState == RunState::PostAnimationCleanup) {
         postAnimationCleanupAndReset();
         return;
@@ -338,14 +363,8 @@ void ScrollAnimator::updateCompositorAnimations()
         if (!m_compositorAnimationAttachedToLayerId)
             reattachCompositorPlayerIfNeeded(getScrollableArea()->compositorAnimationTimeline());
 
-        if (!m_animationCurve) {
-            m_animationCurve = adoptPtr(CompositorFactory::current().createScrollOffsetAnimationCurve(
-                compositorOffsetFromBlinkOffset(m_targetOffset),
-                m_lastGranularity == ScrollByPixel ?
-                    CompositorScrollOffsetAnimationCurve::ScrollDurationInverseDelta :
-                    CompositorScrollOffsetAnimationCurve::ScrollDurationConstant));
-            m_animationCurve->setInitialValue(compositorOffsetFromBlinkOffset(currentPosition()));
-        }
+        if (!m_animationCurve)
+            createAnimationCurve();
 
         bool runningOnMainThread = false;
         bool sentToCompositor = sendAnimationToCompositor();