Add a bit to not bubble scrolls to parent scrolling layers

cc/layer_tree_host_impl.cc

Index: cc/layer_tree_host_impl.cc
diff --git a/cc/layer_tree_host_impl.cc b/cc/layer_tree_host_impl.cc
index 42aa9533c928df6b108bf063cb76bb91c546d130..e016b7557dc6b7f887c41c155164fbe6ee5eef93 100644
--- a/cc/layer_tree_host_impl.cc
+++ b/cc/layer_tree_host_impl.cc
@@ -132,6 +132,8 @@ scoped_ptr<LayerTreeHostImpl> LayerTreeHostImpl::create(const LayerTreeSettings&
 LayerTreeHostImpl::LayerTreeHostImpl(const LayerTreeSettings& settings, LayerTreeHostImplClient* client, Proxy* proxy)
     : m_client(client)
     , m_proxy(proxy)
+    , m_didScrollOnce(false)
+    , m_shouldBubbleScrolls(false)
     , m_scrollDeltaIsInViewportSpace(false)
     , m_settings(settings)
     , m_debugState(settings.initialDebugState)
@@ -1203,6 +1205,7 @@ InputHandlerClient::ScrollStatus LayerTreeHostImpl::scrollBegin(gfx::Point viewp
 
     DCHECK(!currentlyScrollingLayer());
     clearCurrentlyScrollingLayer();
+    m_didScrollOnce = false;

[Sami, 2013/01/30 19:28:31]

Better do this in clearCurrentlyScrollingLayer().

[Yusuf, 2013/01/30 21:01:02]

Done.

 
     if (!ensureRenderSurfaceLayerList())
         return ScrollIgnored;
@@ -1240,6 +1243,7 @@ InputHandlerClient::ScrollStatus LayerTreeHostImpl::scrollBegin(gfx::Point viewp
 
     if (potentiallyScrollingLayerImpl) {
         m_activeTree->set_currently_scrolling_layer(potentiallyScrollingLayerImpl);
+        m_shouldBubbleScrolls = (type != NonBubblingGesture);
         // Gesture events need to be transformed from viewport coordinates to local layer coordinates
         // so that the scrolling contents exactly follow the user's finger. In contrast, wheel
         // events are already in local layer coordinates so we can just apply them directly.
@@ -1341,9 +1345,15 @@ bool LayerTreeHostImpl::scrollBy(const gfx::Point& viewportPoint,
 
         // If the layer wasn't able to move, try the next one in the hierarchy.
         float moveThresholdSquared = 0.1f * 0.1f;
-        if (appliedDelta.LengthSquared() < moveThresholdSquared)
-            continue;
+        if (appliedDelta.LengthSquared() < moveThresholdSquared) {
+            if (m_shouldBubbleScrolls || !m_didScrollOnce)
+                continue;
+            else
+              break;

[Sami, 2013/01/30 19:28:31]

Indent?

[Yusuf, 2013/01/30 21:01:02]

Done.

[Yusuf, 2013/01/30 21:01:02]

Done.

+        }
+        if (!m_shouldBubbleScrolls) m_activeTree->set_currently_scrolling_layer(layerImpl);

[Sami, 2013/01/30 19:28:31]

You could also break here since there's no need to continue recursing, but only
after updating didScroll and m_didScrollOnce.

[Yusuf, 2013/01/30 21:01:02]

Done.

         didScroll = true;
+        m_didScrollOnce = true;
 
         // If the applied delta is within 45 degrees of the input delta, bail out to make it easier
         // to scroll just one layer in one direction without affecting any of its parents.