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

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

 /*
  * Copyright (C) 2011 Apple Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 669 matching lines @@
 {
     m_layersWithTouchRects.remove(layer);
 }
 
 void ScrollingCoordinator::setShouldUpdateScrollLayerPositionOnMainThread(MainThreadScrollingReasons mainThreadScrollingReasons)
 {
     if (!m_page->mainFrame()->isLocalFrame() || !m_page->deprecatedLocalMainFrame()->view())
         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 = ~0u;
+            mainThreadScrollingReasonsToClear &= ~MainThreadScrollingReason::kAnimatingScrollOnMainThread;
+            scrollLayer->clearMainThreadScrollingReasons(mainThreadScrollingReasonsToClear);
+        }
     }
 }
 
 void ScrollingCoordinator::layerTreeViewInitialized(WebLayerTreeView& layerTreeView)
 {
     if (RuntimeEnabledFeatures::compositorAnimationTimelinesEnabled() && Platform::current()->isThreadedAnimationEnabled()) {
         m_programmaticScrollAnimatorTimeline = adoptPtr(CompositorFactory::current().createAnimationTimeline());
         layerTreeView.attachCompositorAnimationTimeline(m_programmaticScrollAnimatorTimeline->animationTimeline());
     }
 }
@@ skipping 322 matching lines @@
 String ScrollingCoordinator::mainThreadScrollingReasonsAsText(MainThreadScrollingReasons reasons)
 {
     StringBuilder stringBuilder;
 
     if (reasons & MainThreadScrollingReason::kHasBackgroundAttachmentFixedObjects)
         stringBuilder.appendLiteral("Has background-attachment:fixed, ");
     if (reasons & MainThreadScrollingReason::kHasNonLayerViewportConstrainedObjects)
         stringBuilder.appendLiteral("Has non-layer viewport-constrained objects, ");
     if (reasons & MainThreadScrollingReason::kThreadedScrollingDisabled)
         stringBuilder.appendLiteral("Threaded scrolling is disabled, ");
+    if (reasons & MainThreadScrollingReason::kAnimatingScrollOnMainThread)
+        stringBuilder.appendLiteral("Animating scroll on main thread, ");
 
     if (stringBuilder.length())
         stringBuilder.resize(stringBuilder.length() - 2);
     return stringBuilder.toString();
 }
 
 String ScrollingCoordinator::mainThreadScrollingReasonsAsText() const
 {
     ASSERT(m_page->deprecatedLocalMainFrame()->document()->lifecycle().state() >= DocumentLifecycle::CompositingClean);
+    if (WebLayer* scrollLayer = toWebLayer(m_page->deprecatedLocalMainFrame()->view()->layerForScrolling()))
+        return mainThreadScrollingReasonsAsText(scrollLayer->mainThreadScrollingReasons());
+
     return mainThreadScrollingReasonsAsText(m_lastMainThreadScrollingReasons);
 }
 
 bool ScrollingCoordinator::frameViewIsDirty() const
 {
     FrameView* frameView = m_page->mainFrame()->isLocalFrame() ? m_page->deprecatedLocalMainFrame()->view() : nullptr;
     bool frameIsScrollable = frameView && frameView->isScrollable();
     if (frameIsScrollable != m_wasFrameScrollable)
         return true;
 
     if (WebLayer* scrollLayer = frameView ? toWebLayer(frameView->layerForScrolling()) : nullptr)
         return WebSize(frameView->contentsSize()) != scrollLayer->bounds();
     return false;
 }
 
 } // namespace blink