Trackpad scrolling could lag on MacOSX

content/browser/renderer_host/input/gesture_event_queue.cc

Index: content/browser/renderer_host/input/gesture_event_queue.cc
diff --git a/content/browser/renderer_host/input/gesture_event_queue.cc b/content/browser/renderer_host/input/gesture_event_queue.cc
index 15cda746b21da1b0c32c55e3ed40d47aad010373..49450f380a70184697c31c3dec93fafe5ac6101f 100644
--- a/content/browser/renderer_host/input/gesture_event_queue.cc
+++ b/content/browser/renderer_host/input/gesture_event_queue.cc
@@ -186,6 +186,9 @@ void GestureEventQueue::QueueAndForwardIfNecessary(
     case WebInputEvent::GestureScrollUpdate:
       QueueScrollOrPinchAndForwardIfNecessary(gesture_event);
       return;
+    case WebInputEvent::GestureScrollBegin:
+      if (FilterScrollBegin(gesture_event))
+        return;
     default:
       break;
   }
@@ -195,6 +198,23 @@ void GestureEventQueue::QueueAndForwardIfNecessary(
     client_->SendGestureEventImmediately(gesture_event);
 }
 
+bool GestureEventQueue::FilterScrollBegin(
+    const GestureEventWithLatencyInfo& gesture_event) {
+  // If a synthetic scroll begin is encountered; it can cancel out a previous

[tdresser, 2016/05/02 13:27:15]

; -> ,

[dtapuska, 2016/05/02 13:57:24]

Done.

+  // synthetic scroll end. This allows a later gesture scroll update to coalesce
+  // with the previous one. crbug.com/607340.
+  bool synthetic = gesture_event.event.data.scrollBegin.synthetic;
+  if (synthetic && EventsInFlightCount() < coalesced_gesture_events_.size()) {

[tdresser, 2016/05/02 13:27:15]

Might be worth factoring out |EventsInFlightCount() <
coalesced_gesture_events_.size()| into a well named bool.

Maybe something like |have_unsent_events|?

[dtapuska, 2016/05/02 13:57:24]

Done.

+    GestureEventWithLatencyInfo* last_event = &coalesced_gesture_events_.back();
+    if (last_event->event.type == WebInputEvent::GestureScrollEnd &&
+        last_event->event.data.scrollEnd.synthetic) {
+      coalesced_gesture_events_.pop_back();
+      return true;
+    }
+  }
+  return false;
+}
+
 void GestureEventQueue::ProcessGestureAck(InputEventAckState ack_result,
                                           WebInputEvent::Type type,
                                           const ui::LatencyInfo& latency) {