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

ui/events/blink/input_handler_proxy.cc

Index: ui/events/blink/input_handler_proxy.cc
diff --git a/ui/events/blink/input_handler_proxy.cc b/ui/events/blink/input_handler_proxy.cc
index 01d2544ceebb997e47f6822a2eea07fb510480f0..54d506e89c58db93fc3c3d8b0e3781fbbe77c7b5 100644
--- a/ui/events/blink/input_handler_proxy.cc
+++ b/ui/events/blink/input_handler_proxy.cc
@@ -797,7 +797,6 @@ InputHandlerProxy::EventDisposition InputHandlerProxy::HandleGestureFlingStart(
                            TRACE_EVENT_SCOPE_THREAD);
       gesture_scroll_on_impl_thread_ = false;
       fling_may_be_active_on_main_thread_ = true;
-      client_->DidStartFlinging();
       return DID_NOT_HANDLE;
     }
     case cc::InputHandler::SCROLL_IGNORED: {
@@ -821,12 +820,14 @@ InputHandlerProxy::EventDisposition InputHandlerProxy::HandleGestureFlingStart(
 InputHandlerProxy::EventDisposition InputHandlerProxy::HandleTouchStart(
     const blink::WebTouchEvent& touch_event) {
   EventDisposition result = DROP_EVENT;
+  cc::InputHandler::TouchStartHitResult hit_result =
+      cc::InputHandler::TouchStartHitResult::SAME_LAYER;
   for (size_t i = 0; i < touch_event.touchesLength; ++i) {
     if (touch_event.touches[i].state != WebTouchPoint::StatePressed)
       continue;
-    if (input_handler_->DoTouchEventsBlockScrollAt(
-            gfx::Point(touch_event.touches[i].position.x,
-                       touch_event.touches[i].position.y))) {
+    hit_result = input_handler_->DoTouchEventsBlockScrollAt(gfx::Point(
+        touch_event.touches[i].position.x, touch_event.touches[i].position.y));
+    if (hit_result != cc::InputHandler::HANDLER) {
       result = DID_NOT_HANDLE;
       break;
     }
@@ -876,7 +877,14 @@ InputHandlerProxy::EventDisposition InputHandlerProxy::HandleTouchStart(
           cc::EventListenerProperties::kNone) {
     result = DID_HANDLE_NON_BLOCKING;
   }
+  bool is_fling_on_impl = fling_curve_ && !fling_may_be_active_on_main_thread_;
+  bool touch_on_active_scroll_layer =
+      hit_result == cc::InputHandler::TouchStartHitResult::SAME_LAYER;
 
+  if (result == DID_NOT_HANDLE && touch_on_active_scroll_layer &&
+      is_fling_on_impl) {
+    result = DID_NOT_HANDLE_NON_BLOCKING;
+  }
   return result;
 }
 
@@ -1301,6 +1309,7 @@ bool InputHandlerProxy::TouchpadFlingScroll(
     case DROP_EVENT:
       break;
     case DID_NOT_HANDLE:
+    case DID_NOT_HANDLE_NON_BLOCKING:
       TRACE_EVENT_INSTANT0("input",
                            "InputHandlerProxy::scrollBy::AbortFling",
                            TRACE_EVENT_SCOPE_THREAD);
@@ -1312,7 +1321,6 @@ bool InputHandlerProxy::TouchpadFlingScroll(
       // the subarea but then is flung "under" the pointer.
       client_->TransferActiveWheelFlingAnimation(fling_parameters_);
       fling_may_be_active_on_main_thread_ = true;
-      client_->DidStartFlinging();
       CancelCurrentFlingWithoutNotifyingClient();
       break;
   }