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

Source/core/page/scrolling/ScrollingCoordinator.cpp

Index: Source/core/page/scrolling/ScrollingCoordinator.cpp
diff --git a/Source/core/page/scrolling/ScrollingCoordinator.cpp b/Source/core/page/scrolling/ScrollingCoordinator.cpp
index a1b1ee214807db01d3a9195c435bbbd7937d4147..e54d9b4d502314a27b0474dc4f03a33256387bb3 100644
--- a/Source/core/page/scrolling/ScrollingCoordinator.cpp
+++ b/Source/core/page/scrolling/ScrollingCoordinator.cpp
@@ -74,6 +74,13 @@ 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);
@@ -153,7 +160,7 @@ void ScrollingCoordinator::updateAfterCompositingChange()
     }
 
     FrameView* frameView = m_page->mainFrame()->view();
-    bool frameIsScrollable = frameView && frameView->isScrollable();
+    bool frameIsScrollable = frameViewCouldBeScrolledByImplThread(frameView);
     if (m_wasFrameScrollable != frameIsScrollable)
         updateShouldUpdateScrollLayerPositionOnMainThread();
     m_wasFrameScrollable = frameIsScrollable;
@@ -615,6 +622,8 @@ void ScrollingCoordinator::setShouldUpdateScrollLayerPositionOnMainThread(MainTh
         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()
@@ -883,7 +892,7 @@ MainThreadScrollingReasons ScrollingCoordinator::mainThreadScrollingReasons() co
     // 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;
@@ -925,7 +934,7 @@ String ScrollingCoordinator::mainThreadScrollingReasonsAsText() const
 bool ScrollingCoordinator::frameViewIsScrollableIsDirty() const
 {
     FrameView* frameView = m_page->mainFrame()->view();
-    bool frameIsScrollable = frameView && frameView->isScrollable();
+    bool frameIsScrollable = frameViewCouldBeScrolledByImplThread(frameView);
     return frameIsScrollable != m_wasFrameScrollable;
 }