Deprecate use of user gestures during scroll-related touch events

content/browser/renderer_host/input/input_router_impl.cc

Index: content/browser/renderer_host/input/input_router_impl.cc
diff --git a/content/browser/renderer_host/input/input_router_impl.cc b/content/browser/renderer_host/input/input_router_impl.cc
index e6ddcd9ed7f0604b966f382d39266f4c7ed7f984..aee5c790312e7dc4df10be8465c4e4c8bca47332 100644
--- a/content/browser/renderer_host/input/input_router_impl.cc
+++ b/content/browser/renderer_host/input/input_router_impl.cc
@@ -83,6 +83,7 @@ InputRouterImpl::InputRouterImpl(IPC::Sender* sender,
       current_ack_source_(ACK_SOURCE_NONE),
       flush_requested_(false),
       active_renderer_fling_count_(0),
+      touch_scroll_started_sent_(false),
       wheel_event_queue_(this,
                          UseGestureBasedWheelScrolling(),
                          kDefaultWheelScrollTransactionMs),
@@ -161,8 +162,18 @@ void InputRouterImpl::SendGestureEvent(
   wheel_event_queue_.OnGestureScrollEvent(gesture_event);
 
   if (gesture_event.event.sourceDevice == blink::WebGestureDeviceTouchscreen) {
-    if (gesture_event.event.type == blink::WebInputEvent::GestureScrollBegin)
+    if (gesture_event.event.type == blink::WebInputEvent::GestureScrollBegin) {
+      touch_scroll_started_sent_ = false;
+    } else if(!touch_scroll_started_sent_
+        && gesture_event.event.type ==
+            blink::WebInputEvent::GestureScrollUpdate) {
+      // A touch scroll hasn't really started until the first
+      // GestureScrollUpdate event.  Eg. if the page consumes all touchmoves
+      // then no scrolling really ever occurs (even though we still send
+      // GestureScrollBegin).
       touch_event_queue_.PrependTouchScrollNotification();

[mustaq, 2016/05/24 15:54:05]

Originally when PrependTouchScrollNotification() was called, we used to have an
in-flight touchmove corresponding to the first touchmove beyond the slop region.
Now that the TSS event is delayed by one step, am I right that the pointercancel
event on a touch-action:none region will be seen after 2 touchmoves? What if a
2nd finger touchstart happens between the two touchmoves, so there will be no
GSU hence no TSS?

These seem insignificant but wanted to call out in case they may trigger some
observable regression somewhere.

[Rick Byers, 2016/05/24 20:29:21]

No, in the common case the first GestureScrollUpdate follows immediately the
GestureScrollBegin (both generated from the same touchmove event).  See
https://code.google.com/p/chromium/codesearch#chromium/src/ui/events/gesture_....
 So there's nothing that should ever happen between the GestureScrollBegin and
the first GestureScrollUpdate.

Also note that in the touch-action: none case there is no TSS / pointercancel
(because no GestureScrollBegin/Update is sent to the renderer at all - they're
entirely filtered out by the TouchActionGestureFilter).

+      touch_scroll_started_sent_ = true;
+    }
     touch_event_queue_.OnGestureScrollEvent(gesture_event);
   }