Make touch events uncancelable during fling when they are on the current active scroll layer

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 2f044eeab78af6ffb4d94972f5fc60b810aac68a..d4c604b8cc64bdce580c710ffa0ef1f240afde06 100644
--- a/cc/trees/layer_tree_host_impl.cc
+++ b/cc/trees/layer_tree_host_impl.cc
@@ -155,6 +155,24 @@ void RecordCompositorSlowScrollMetric(InputHandler::ScrollInputType type,
   }
 }
 
+// Return true if scroll_ancestor is same as child or its ancestor.
+bool isAncestor(LayerImpl* child, LayerImpl* scroll_ancestor) {

[bokan, 2016/11/11 20:41:53]

Lets replace HasScrollAncestor with this function.

+  DCHECK(scroll_ancestor);
+  if (!child)
+    return false;
+  if (child->id() == scroll_ancestor->id())
+    return true;
+  ScrollTree& scroll_tree =
+      child->layer_tree_impl()->property_trees()->scroll_tree;
+  ScrollNode* scroll_node = scroll_tree.Node(child->scroll_tree_index());

[bokan, 2016/11/11 20:41:53]

Put this the for loop initializer below:

for(ScrollNode* scroll_node = ...; )

[lanwei, 2016/11/12 02:17:15]

Done.

+  for (; scroll_tree.parent(scroll_node);

[bokan, 2016/11/11 20:41:53]

This is still only looping if we have a parent which will miss the root node.
E.g:

A <- B <- C

If child = C and scroll_ancestor = A:

Iteration 1: Compare C and A, set scroll_node = B (side note: we've already
compared C and A above, you can remove the child == scroll_ancestor check above)
Iteration 2: Compare B and A, set scroll_node = A

But now scroll_tree.parent(A) == nullptr so we won't iterate again. We should be
looping while scroll_node is non-null.

[lanwei, 2016/11/12 02:17:15]

Done.

+       scroll_node = scroll_tree.parent(scroll_node)) {
+    if (scroll_node->owner_id == scroll_ancestor->id())
+      return true;
+  }
+  return false;
+}
+
 }  // namespace
 
 DEFINE_SCOPED_UMA_HISTOGRAM_TIMER(PendingTreeDurationHistogramTimer,
@@ -582,7 +600,7 @@ EventListenerProperties LayerTreeHostImpl::GetEventListenerProperties(
   return active_tree_->event_listener_properties(event_class);
 }
 
-bool LayerTreeHostImpl::DoTouchEventsBlockScrollAt(
+EventListenerProperties LayerTreeHostImpl::DoTouchHandlersBlockScrollAt(
     const gfx::Point& viewport_point) {
   gfx::PointF device_viewport_point = gfx::ScalePoint(
       gfx::PointF(viewport_point), active_tree_->device_scale_factor());
@@ -592,7 +610,16 @@ bool LayerTreeHostImpl::DoTouchEventsBlockScrollAt(
   LayerImpl* layer_impl =
       active_tree_->FindLayerThatIsHitByPointInTouchHandlerRegion(
           device_viewport_point);
-  return layer_impl != NULL;
+  if (layer_impl == NULL)
+    return EventListenerProperties::kNone;
+
+  if (!CurrentlyScrollingLayer())
+    return EventListenerProperties::kBlocking;
+
+  bool is_ancestor =
+      isAncestor(layer_impl, active_tree_->CurrentlyScrollingLayer());
+  return is_ancestor ? EventListenerProperties::kBlockingAndPassiveDueToFling
+                     : EventListenerProperties::kBlocking;
 }
 
 std::unique_ptr<SwapPromiseMonitor>
@@ -2683,7 +2710,6 @@ InputHandler::ScrollStatus LayerTreeHostImpl::ScrollBegin(
 
   if (scroll_on_main_thread) {
     RecordCompositorSlowScrollMetric(type, MAIN_THREAD);
-
     scroll_status.thread = SCROLL_ON_MAIN_THREAD;
     return scroll_status;
   }