Touchpad and wheel scroll latching for ChromeOS behind flag.

content/browser/renderer_host/input/mouse_wheel_event_queue.cc

Index: content/browser/renderer_host/input/mouse_wheel_event_queue.cc
diff --git a/content/browser/renderer_host/input/mouse_wheel_event_queue.cc b/content/browser/renderer_host/input/mouse_wheel_event_queue.cc
index 113fb42dfd9320b48cc975863a8423fe7ec313cf..0d92d81583175a3b6dd5283d9166ad93c27ad320 100644
--- a/content/browser/renderer_host/input/mouse_wheel_event_queue.cc
+++ b/content/browser/renderer_host/input/mouse_wheel_event_queue.cc
@@ -278,8 +278,17 @@ void MouseWheelEventQueue::OnGestureScrollEvent(
               gesture_event.event.type ==
                   blink::WebInputEvent::GestureFlingStart)) {
     scrolling_device_ = blink::WebGestureDeviceUninitialized;
-    if (scroll_end_timer_.IsRunning())
-      scroll_end_timer_.Reset();
+    if (scroll_end_timer_.IsRunning()) {

[tdresser, 2017/01/09 18:25:19]

This should only happen if latching is enabled, right? Can we DCHECK that
latching is enabled?

[sahel, 2017/01/09 20:02:01]

If the latching is not enabled then the timeout for the timer will be zero, is
it possible (in some rare cases) to have the timer running when Fling arrives?
If not, then you are right we can add a DCHECK and get rid of the else part.

[tdresser, 2017/01/10 16:19:02]

I missed that we do start the timer when latching is disabled.

This LGTM.

[bokan, 2017/01/10 18:53:58]

I don't actually know how we implement task queues, but I'd assume that a
0-delay timer would get queued but might be behind an already queued FlingStart.
(unless they're in separate queues and timers get priority)

Perhaps if latching is disabled, rather than queuing the 0-delay timer, we
should just synchronously run it?

[sahel, 2017/01/10 21:10:06]

I don't know if timer tasks have priority or not, maybe @tdresser can help with
the answer.
The timer starts even when the latching is disabled. I don't know if we always
want to have zero timeout in this case or not, if the answer is yes, then we can
get rid of the timer when latching is disabled.

+      if (enable_scroll_latching_) {
+        // Don't send the pending ScrollEnd if a fling is happening.
+        // The next wheel event will still need a ScrollBegin.
+        scroll_end_timer_.Stop();
+        needs_scroll_begin_ = true;

[bokan, 2017/01/10 18:53:58]

If the event is a GFS, do we still need a GSB?

[sahel, 2017/01/10 21:10:06]

Yes, this is for making the gesture event sequences for touchscreen and touchpad
scrolls similar.
For touchpad we will have GSB (GSU)xn GFS
we won't send a GSE after/before GFS and that's why needs_scroll_begin_ is set
to true directly for the next GSU. The next GSU won't be for this scroll
sequence because when we are handling touchpad fling, we don't generate GSU
events after the fling. 

[bokan, 2017/01/10 21:25:06]

Ah, ok. Thanks for the explanation.

+        needs_scroll_end_ = false;
+      } else {
+        scroll_end_timer_.Reset();
+      }
+    }
   }
 }