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 1c60e798136e1599d8360977ec278348159225d0..6847ef690ea0d2f16c25c0dae11171c00692c524 100644
--- a/cc/layer_tree_host_impl.cc
+++ b/cc/layer_tree_host_impl.cc
@@ -132,6 +132,7 @@ 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_scrollDeltaIsInViewportSpace(false)
     , m_settings(settings)
     , m_debugState(settings.initialDebugState)
@@ -1174,6 +1175,7 @@ InputHandlerClient::ScrollStatus LayerTreeHostImpl::scrollBegin(gfx::Point viewp
 
     DCHECK(!currentlyScrollingLayer());
     clearCurrentlyScrollingLayer();
+    m_didScrollOnce = false;
 
     if (!ensureRenderSurfaceLayerList())
         return ScrollIgnored;
@@ -1287,7 +1289,8 @@ static gfx::Vector2dF scrollLayerWithLocalDelta(LayerImpl& layerImpl, gfx::Vecto
 }
 
 bool LayerTreeHostImpl::scrollBy(const gfx::Point& viewportPoint,
-                                 const gfx::Vector2d& scrollDelta)
+                                 const gfx::Vector2d& scrollDelta,
+                                 bool shouldBubble)
 {
     TRACE_EVENT0("cc", "LayerTreeHostImpl::scrollBy");
     if (!currentlyScrollingLayer())
@@ -1312,9 +1315,14 @@ 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 (shouldBubble || !m_didScrollOnce)

[Sami, 2013/01/29 11:42:34]

I'm not sure this is correct. When we're not bubbling, this seems to allow
scrollBy to bubble once up to a layer that can scroll. The next time scrollBy is
called, it terminates at the first layer that couldn't scroll.

Let's say you have a page with an overflow div that was already scrolled all the
way down. Flinging that div further down should fling the page instead. I think
with this code that wouldn't happen.

[Yusuf, 2013/01/29 18:59:21]

Yes, I think you are right. I somehow thought the current_scrolling_layer gets
updated when we bubble up here, so that we remember where we left of, but that
doesn't look like the case. I might need to keep the layer itself like we had
talked before.

On 2013/01/29 11:42:34, Sami wrote:

+                continue;
+            else
+              break;

[Sami, 2013/01/29 11:42:34]

Indentation seem off.

+        }
         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.