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 ead5db73d6b313687d5e2193cc69a19024c1293d..c8104e97930bdd04bcf6df2e72d7c2a1c65cb23d 100644
--- a/cc/trees/layer_tree_host_impl.cc
+++ b/cc/trees/layer_tree_host_impl.cc
@@ -155,6 +155,22 @@ void RecordCompositorSlowScrollMetric(InputHandler::ScrollInputType type,
   }
 }
 
+// Similar to LayerImpl::HasAncestor, but walks up the scroll parents.
+static bool HasScrollAncestor(LayerImpl* child, LayerImpl* scroll_ancestor) {
+  DCHECK(scroll_ancestor);
+  if (!child)
+    return false;
+  ScrollTree& scroll_tree =
+      child->layer_tree_impl()->property_trees()->scroll_tree;
+  ScrollNode* scroll_node = scroll_tree.Node(child->scroll_tree_index());
+  for (; scroll_tree.parent(scroll_node);

[bokan, 2016/11/04 19:32:55]

This will avoid comparing against the root layer. I would change this to just
check that scroll_node is non-null

+       scroll_node = scroll_tree.parent(scroll_node)) {
+    if (scroll_node->scrollable)
+      return scroll_node->owner_id == scroll_ancestor->id();
+  }
+  return false;
+}
+
 }  // namespace
 
 DEFINE_SCOPED_UMA_HISTOGRAM_TIMER(PendingTreeDurationHistogramTimer,
@@ -591,7 +607,7 @@ EventListenerProperties LayerTreeHostImpl::GetEventListenerProperties(
   return active_tree_->event_listener_properties(event_class);
 }
 
-bool LayerTreeHostImpl::DoTouchEventsBlockScrollAt(
+InputHandler::TouchStartHitResult LayerTreeHostImpl::DoTouchEventsBlockScrollAt(
     const gfx::Point& viewport_point) {
   gfx::PointF device_viewport_point = gfx::ScalePoint(
       gfx::PointF(viewport_point), active_tree_->device_scale_factor());
@@ -601,7 +617,16 @@ bool LayerTreeHostImpl::DoTouchEventsBlockScrollAt(
   LayerImpl* layer_impl =
       active_tree_->FindLayerThatIsHitByPointInTouchHandlerRegion(
           device_viewport_point);
-  return layer_impl != NULL;
+  if (layer_impl == NULL)
+    return TouchStartHitResult::HANDLER;
+
+  if (!CurrentlyScrollingLayer())
+    return TouchStartHitResult::SAME_LAYER;

[tdresser, 2016/11/04 17:15:20]

I don't understand this, but I might just be confused by the naming.

[bokan, 2016/11/04 19:32:55]

+1, I don't know what these returns mean.

+
+  bool is_ancestor =
+      HasScrollAncestor(layer_impl, active_tree_->CurrentlyScrollingLayer());

[bokan, 2016/11/04 19:32:55]

What should happen if layer_impl == CurrentlyScrollingLayer? HasScrollAncestor
will return false but maybe that's by design?

+  return is_ancestor ? TouchStartHitResult::SAME_LAYER
+                     : TouchStartHitResult::DIFFERENT_LAYER;
 }
 
 std::unique_ptr<SwapPromiseMonitor>
@@ -2575,22 +2600,6 @@ LayerImpl* LayerTreeHostImpl::FindScrollLayerForDeviceViewportPoint(
   return potentially_scrolling_layer_impl;
 }
 
-// Similar to LayerImpl::HasAncestor, but walks up the scroll parents.
-static bool HasScrollAncestor(LayerImpl* child, LayerImpl* scroll_ancestor) {
-  DCHECK(scroll_ancestor);
-  if (!child)
-    return false;
-  ScrollTree& scroll_tree =
-      child->layer_tree_impl()->property_trees()->scroll_tree;
-  ScrollNode* scroll_node = scroll_tree.Node(child->scroll_tree_index());
-  for (; scroll_tree.parent(scroll_node);
-       scroll_node = scroll_tree.parent(scroll_node)) {
-    if (scroll_node->scrollable)
-      return scroll_node->owner_id == scroll_ancestor->id();
-  }
-  return false;
-}
-
 InputHandler::ScrollStatus LayerTreeHostImpl::ScrollBeginImpl(
     ScrollState* scroll_state,
     LayerImpl* scrolling_layer_impl,
@@ -2685,7 +2694,6 @@ InputHandler::ScrollStatus LayerTreeHostImpl::ScrollBegin(
 
   if (scroll_on_main_thread) {
     RecordCompositorSlowScrollMetric(type, MAIN_THREAD);
-
     scroll_status.thread = SCROLL_ON_MAIN_THREAD;
     return scroll_status;
   }