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

cc/trees/layer_tree_host_impl.cc

 // Copyright 2011 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "cc/trees/layer_tree_host_impl.h"
 
 #include <stddef.h>
 #include <stdint.h>
 
 #include <algorithm>
@@ skipping 137 matching lines @@
   bool scroll_on_main_thread = (scroll_thread == MAIN_THREAD);
   if (IsWheelBasedScroll(type)) {
     UMA_HISTOGRAM_BOOLEAN("Renderer4.CompositorWheelScrollUpdateThread",
                           scroll_on_main_thread);
   } else {
     UMA_HISTOGRAM_BOOLEAN("Renderer4.CompositorTouchScrollUpdateThread",
                           scroll_on_main_thread);
   }
 }
 
+// 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,
                                   "Scheduling.%s.PendingTreeDuration");
 
 LayerTreeHostImpl::FrameData::FrameData()
     : render_surface_layer_list(nullptr),
       has_no_damage(false),
       may_contain_video(false) {}
 
@@ skipping 407 matching lines @@
   }
 
   return false;
 }
 
 EventListenerProperties LayerTreeHostImpl::GetEventListenerProperties(
     EventListenerClass event_class) const {
   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());
 
   // Now determine if there are actually any handlers at that point.
   // TODO(rbyers): Consider also honoring touch-action (crbug.com/347272).
   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>
 LayerTreeHostImpl::CreateLatencyInfoSwapPromiseMonitor(
     ui::LatencyInfo* latency) {
   return base::WrapUnique(
       new LatencyInfoSwapPromiseMonitor(latency, NULL, this));
 }
 
 ScrollElasticityHelper* LayerTreeHostImpl::CreateScrollElasticityHelper() {
@@ skipping 2070 matching lines @@
   LayerImpl* scrolling_layer_impl = FindScrollLayerForDeviceViewportPoint(
       device_viewport_point, type, layer_impl, &scroll_on_main_thread,
       &scroll_status.main_thread_scrolling_reasons);
 
   if (scrolling_layer_impl)
     scroll_affects_scroll_handler_ =
         scrolling_layer_impl->layer_tree_impl()->have_scroll_event_handlers();
 
   if (scroll_on_main_thread) {
     RecordCompositorSlowScrollMetric(type, MAIN_THREAD);
-
     scroll_status.thread = SCROLL_ON_MAIN_THREAD;
     return scroll_status;
   }
 
   return ScrollBeginImpl(scroll_state, scrolling_layer_impl, type);
 }
 
 InputHandler::ScrollStatus LayerTreeHostImpl::ScrollAnimatedBegin(
     const gfx::Point& viewport_point) {
   InputHandler::ScrollStatus scroll_status;
@@ skipping 1388 matching lines @@
   if (is_visible) {
     worker_context_visibility_ =
         worker_context->CacheController()->ClientBecameVisible();
   } else {
     worker_context->CacheController()->ClientBecameNotVisible(
         std::move(worker_context_visibility_));
   }
 }
 
 }  // namespace cc