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

third_party/WebKit/Source/platform/graphics/GraphicsLayer.cpp

Index: third_party/WebKit/Source/platform/graphics/GraphicsLayer.cpp
diff --git a/third_party/WebKit/Source/platform/graphics/GraphicsLayer.cpp b/third_party/WebKit/Source/platform/graphics/GraphicsLayer.cpp
index 2ba8104216248a8ce57dea7de4b3e321142a4e0f..89377fea074f6c16699764812283af77c2985adc 100644
--- a/third_party/WebKit/Source/platform/graphics/GraphicsLayer.cpp
+++ b/third_party/WebKit/Source/platform/graphics/GraphicsLayer.cpp
@@ -1174,6 +1174,25 @@ void GraphicsLayer::removeLinkHighlight(LinkHighlight* linkHighlight)
     updateChildList();
 }
 
+void GraphicsLayer::setMainThreadScrollingReasons(uint32_t reasons)
+{
+    if (!reasons && m_scrollableArea) {
+        // There may be a scroll animation running on the main thread. Forward
+        // the call to the ScrollAnimator that will clear the main thread
+        // scrolling reasons after the animation is complete.
+        m_scrollableArea->clearMainThreadScrollingReasons(true);
+        return;
+    }
+    if (WebLayer* scrollLayer = platformLayer()) {
+        // The ScrollAnimator will override this main thread scrolling reason if
+        // it is added while the ScrollAnimator is scheduled to clear the main
+        // thread scrolling reason (from call above). Thus, inform it to not
+        // clear the main thread scrolling reasons.
+        m_scrollableArea->clearMainThreadScrollingReasons(false);
+        scrollLayer->addMainThreadScrollingReasons(reasons);

[ajuma, 2016/02/04 15:05:48]

Would it simplify things if the main thread scroll animator always adds a main
thread scrolling reason when it starts an animation (or when a composited layer
is created during an animation;
ScrollAnimator::layerForCompositedScrollingDidChange gets called in that case)
and removes this reason when it finishes? Then other code is free to add/remove
other reasons independently, without having to tell the scrollable area.

+    }
+}
+
 void GraphicsLayer::setScrollableArea(ScrollableArea* scrollableArea, bool isViewport)
 {
     if (m_scrollableArea == scrollableArea)