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

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 3a64da20ff27cc8540beb7a5f16fa4fe72774714..08995a430ac8fca7eea22fcd79aa36bb8e61aab6 100644
--- a/third_party/WebKit/Source/core/page/scrolling/ScrollingCoordinator.cpp
+++ b/third_party/WebKit/Source/core/page/scrolling/ScrollingCoordinator.cpp
@@ -686,10 +686,15 @@ void ScrollingCoordinator::setShouldUpdateScrollLayerPositionOnMainThread(MainTh
         return;
     if (WebLayer* scrollLayer = toWebLayer(m_page->deprecatedLocalMainFrame()->view()->layerForScrolling())) {
         m_lastMainThreadScrollingReasons = mainThreadScrollingReasons;
-        if (mainThreadScrollingReasons)
+        if (mainThreadScrollingReasons) {
             scrollLayer->addMainThreadScrollingReasons(mainThreadScrollingReasons);
-        else
-            scrollLayer->clearMainThreadScrollingReasons();
+        } else {
+            // Clear all main thread scrolling reasons except the one that's set
+            // if there is a running scroll animation.
+            uint32_t mainThreadScrollingReasonsToClear = ~0;

[skobes, 2016/02/04 20:04:40]

I had to change this to ~0u to compile on Windows.

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

Thanks! Fixed.

+            mainThreadScrollingReasonsToClear &= ~MainThreadScrollingReason::kAnimatingScollOnMainThread;
+            scrollLayer->clearMainThreadScrollingReasons(mainThreadScrollingReasonsToClear);
+        }
     }
 }