Respect main thread scrolling reasons for fallback to root layer scrolling.

third_party/WebKit/Source/core/page/scrolling/ScrollingCoordinator.cpp

Index: third_party/WebKit/Source/core/page/scrolling/ScrollingCoordinator.cpp
diff --git a/third_party/WebKit/Source/core/page/scrolling/ScrollingCoordinator.cpp b/third_party/WebKit/Source/core/page/scrolling/ScrollingCoordinator.cpp
index 4b444f9b9237fa328f1dcbec4003f9b4d6c1f077..98788a98b0c73df40eaa0ba0f384a2d9d5be4d19 100644
--- a/third_party/WebKit/Source/core/page/scrolling/ScrollingCoordinator.cpp
+++ b/third_party/WebKit/Source/core/page/scrolling/ScrollingCoordinator.cpp
@@ -671,6 +671,7 @@ void ScrollingCoordinator::setShouldUpdateScrollLayerPositionOnMainThread(MainTh
     if (!m_page->mainFrame()->isLocalFrame() || !m_page->deprecatedLocalMainFrame()->view())
         return;
 
+    WebLayer* visualViewportScrollLayer = toWebLayer(m_page->frameHost().visualViewport().scrollLayer());
     GraphicsLayer* layer = m_page->deprecatedLocalMainFrame()->view()->layerForScrolling();
     if (WebLayer* scrollLayer = toWebLayer(layer)) {
         m_lastMainThreadScrollingReasons = mainThreadScrollingReasons;
@@ -680,12 +681,16 @@ void ScrollingCoordinator::setShouldUpdateScrollLayerPositionOnMainThread(MainTh
                 scrollAnimator->takeOverCompositorAnimation();
             }
             scrollLayer->addMainThreadScrollingReasons(mainThreadScrollingReasons);
+            if (visualViewportScrollLayer)
+                visualViewportScrollLayer->addMainThreadScrollingReasons(mainThreadScrollingReasons);

[bokan, 2016/07/01 19:02:28]

I think we need to do the above animation takeover for the VisualViewport but I
don't understand this code well. +ymalik@ to confirm

[Eric Seckler, 2016/07/04 08:33:26]

Seems like you're right. Added the take-over, but will wait for feedback from
ymalik@ before committing :)

[ymalik, 2016/07/04 13:52:07]

Yes, the takeover for the visual viewport will ensure that any impl-only scroll
offset animation is completed on main thread.

         } else {
             // Clear all main thread scrolling reasons except the one that's set
             // if there is a running scroll animation.
             uint32_t mainThreadScrollingReasonsToClear = ~0u;
             mainThreadScrollingReasonsToClear &= ~MainThreadScrollingReason::kAnimatingScrollOnMainThread;
             scrollLayer->clearMainThreadScrollingReasons(mainThreadScrollingReasonsToClear);
+            if (visualViewportScrollLayer)
+                visualViewportScrollLayer->clearMainThreadScrollingReasons(mainThreadScrollingReasonsToClear);
         }
     }
 }