Ensure that if FrameView::isScrollable() is false, the assoc WebLayer is, too.

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 56 matching lines @@
 using blink::WebVector;
 
 
 namespace WebCore {
 
 static WebLayer* scrollingWebLayerForGraphicsLayer(GraphicsLayer* layer)
 {
     return layer->platformLayer();
 }
 
+static bool frameViewCouldBeScrolledByImplThread(FrameView* frameView)
+{
+    // A frame view will report that isScrollable is false if overflow is hidden, but it may still
+    // end up getting scrolled on the impl thread.
+    return frameView && frameView->hasOverflow() && frameView->isVisibleToHitTesting();
+}
+
 WebLayer* ScrollingCoordinator::scrollingWebLayerForScrollableArea(ScrollableArea* scrollableArea)
 {
     GraphicsLayer* graphicsLayer = scrollLayerForScrollableArea(scrollableArea);
     return graphicsLayer ? scrollingWebLayerForGraphicsLayer(graphicsLayer) : 0;
 }
 
 PassRefPtr<ScrollingCoordinator> ScrollingCoordinator::create(Page* page)
 {
     return adoptRef(new ScrollingCoordinator(page));
 }
@@ skipping 59 matching lines @@
         setShouldHandleScrollGestureOnMainThreadRegion(shouldHandleScrollGestureOnMainThreadRegion);
         m_scrollGestureRegionIsDirty = false;
     }
 
     if (m_touchEventTargetRectsAreDirty) {
         updateTouchEventTargetRectsIfNeeded();
         m_touchEventTargetRectsAreDirty = false;
     }
 
     FrameView* frameView = m_page->mainFrame()->view();
-    bool frameIsScrollable = frameView && frameView->isScrollable();
+    bool frameIsScrollable = frameViewCouldBeScrolledByImplThread(frameView);
     if (m_wasFrameScrollable != frameIsScrollable)
         updateShouldUpdateScrollLayerPositionOnMainThread();
     m_wasFrameScrollable = frameIsScrollable;
 
     const FrameTree& tree = m_page->mainFrame()->tree();
     for (const Frame* child = tree.firstChild(); child; child = child->tree().nextSibling()) {
         if (WebLayer* scrollLayer = scrollingWebLayerForScrollableArea(child->view()))
             scrollLayer->setBounds(child->view()->contentsSize());
     }
 }
@@ skipping 441 matching lines @@
 {
     setWheelEventHandlerCount(computeCurrentWheelEventHandlerCount());
 }
 
 void ScrollingCoordinator::setShouldUpdateScrollLayerPositionOnMainThread(MainThreadScrollingReasons reasons)
 {
     if (WebLayer* scrollLayer = scrollingWebLayerForScrollableArea(m_page->mainFrame()->view())) {
         m_lastMainThreadScrollingReasons = reasons;
         scrollLayer->setShouldScrollOnMainThread(reasons);
     }
+
+    // We may be ignoring main thread scrolling reasons if the main thread claims it cannot be scrolled. Check for this case.

[shawnsingh, 2013/12/17 09:05:11]

misplaced comment?  or should this be labeled as a FIXME? or am I just confused?

 }
 
 void ScrollingCoordinator::pageDestroyed()
 {
     ASSERT(m_page);
     m_page = 0;
 }
 
 bool ScrollingCoordinator::coordinatesScrollingForFrameView(FrameView* frameView) const
 {
@@ skipping 248 matching lines @@
     }
     return false;
 }
 
 MainThreadScrollingReasons ScrollingCoordinator::mainThreadScrollingReasons() const
 {
     // The main thread scrolling reasons are applicable to scrolls of the main
     // frame. If it does not exist or if it is not scrollable, there is no
     // reason to force main thread scrolling.
     FrameView* frameView = m_page->mainFrame()->view();
-    if (!frameView || !frameView->isScrollable())
+    if (!frameView || !frameViewCouldBeScrolledByImplThread(frameView))
         return static_cast<MainThreadScrollingReasons>(0);
 
     MainThreadScrollingReasons mainThreadScrollingReasons = (MainThreadScrollingReasons)0;
 
     if (frameView->hasSlowRepaintObjects())
         mainThreadScrollingReasons |= HasSlowRepaintObjects;
     if (hasVisibleSlowRepaintViewportConstrainedObjects(frameView))
         mainThreadScrollingReasons |= HasNonLayerViewportConstrainedObjects;
 
     return mainThreadScrollingReasons;
@@ skipping 21 matching lines @@
 }
 
 String ScrollingCoordinator::mainThreadScrollingReasonsAsText() const
 {
     return mainThreadScrollingReasonsAsText(m_lastMainThreadScrollingReasons);
 }
 
 bool ScrollingCoordinator::frameViewIsScrollableIsDirty() const
 {
     FrameView* frameView = m_page->mainFrame()->view();
-    bool frameIsScrollable = frameView && frameView->isScrollable();
+    bool frameIsScrollable = frameViewCouldBeScrolledByImplThread(frameView);
     return frameIsScrollable != m_wasFrameScrollable;
 }
 
 } // namespace WebCore