Wheel scroll latching enabled behind flag.

cc/trees/layer_tree_host_impl.cc

Index: cc/trees/layer_tree_host_impl.cc
diff --git a/cc/trees/layer_tree_host_impl.cc b/cc/trees/layer_tree_host_impl.cc
index e6db70e6cbf14ee4bb5643da1ca6ed8efd3569dc..2a9e01ff2c265eb162282cc9bd68659b134eae0f 100644
--- a/cc/trees/layer_tree_host_impl.cc
+++ b/cc/trees/layer_tree_host_impl.cc
@@ -2684,8 +2684,12 @@ InputHandler::ScrollStatus LayerTreeHostImpl::ScrollBegin(
   ScrollStatus scroll_status;
   scroll_status.main_thread_scrolling_reasons =
       MainThreadScrollingReason::kNotScrollingOnMain;
+
   TRACE_EVENT0("cc", "LayerTreeHostImpl::ScrollBegin");
 
+  if (scroll_state->is_in_inertial_phase())
+    return FlingScrollBegin();

[ericrk, 2016/07/19 20:21:51]

To make sure I'm understanding this:

Is the idea that, in the case of is_in_inertial_phase, we want to behave as
though this is a fling, not a scroll?

Can you add a comment here?

[sahel, 2016/07/19 22:47:03]

Yes, because in mac flings are handled as scrolls.
I added the comment.


Done.

+
   ClearCurrentlyScrollingLayer();
 
   gfx::Point viewport_point(scroll_state->position_x(),
@@ -3238,7 +3242,13 @@ void LayerTreeHostImpl::ScrollEnd(ScrollState* scroll_state) {
 
   DistributeScrollDelta(scroll_state);
   top_controls_manager_->ScrollEnd();
-  ClearCurrentlyScrollingLayer();
+
+  if (scroll_state->is_in_inertial_phase()) {

[ericrk, 2016/07/19 20:21:51]

I though that is_in_inertial_phase indicated that we had a new fling? I'm
probably just misunderstanding, but it seems like we are clearing the currently
scrolling layer iff we have a new fling?

[sahel, 2016/07/19 22:47:03]

in ScrollEnd, is_in_inertial_phase is true iff the fling is happening. The idea
is not to clear the scrolling layer in cases that we are in the end of a scroll
because a fling might happen afterwards and we don't want a new hit test for the
fling that happens after a scroll.

It's safe not to clear scrolling layer, because we clear it in scrollBegin. 

I can remove the condition and never clear the scrollinglayer in this function.
But after a fling we know that it's the end of current gesture sequence and
anything that happens after needs a new hit testing. So, it is safe to clear the
scrollinglayer.

[ericrk, 2016/07/20 17:44:35]

Ok, this makes sense, thanks for explaining.

+    // There might be a fling after the scroll end. In those cases,
+    // don't clear the currently scrolling layer to avoid a new hit test for the
+    // fling.
+    ClearCurrentlyScrollingLayer();
+  }
 }
 
 InputHandler::ScrollStatus LayerTreeHostImpl::FlingScrollBegin() {