Trackpad scrolling could lag on MacOSX

content/browser/renderer_host/input/gesture_event_queue.cc

 // Copyright 2014 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 "content/browser/renderer_host/input/gesture_event_queue.h"
 
 #include "base/trace_event/trace_event.h"
 #include "content/browser/renderer_host/input/touchpad_tap_suppression_controller.h"
 #include "content/browser/renderer_host/input/touchscreen_tap_suppression_controller.h"
 
@@ skipping 168 matching lines @@
     case WebInputEvent::GestureFlingCancel:
       fling_in_progress_ = false;
       break;
     case WebInputEvent::GestureFlingStart:
       fling_in_progress_ = true;
       break;
     case WebInputEvent::GesturePinchUpdate:
     case WebInputEvent::GestureScrollUpdate:
       QueueScrollOrPinchAndForwardIfNecessary(gesture_event);
       return;
+    case WebInputEvent::GestureScrollBegin:
+      if (OnScrollBegin(gesture_event))
+        return;
     default:
       break;
   }
 
   coalesced_gesture_events_.push_back(gesture_event);
   if (coalesced_gesture_events_.size() == 1)
     client_->SendGestureEventImmediately(gesture_event);
 }
 
+bool GestureEventQueue::OnScrollBegin(
+    const GestureEventWithLatencyInfo& gesture_event) {
+  // If a synthetic scroll begin is encountered, it can cancel out a previous
+  // 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;
+  bool have_unsent_events =
+      EventsInFlightCount() < coalesced_gesture_events_.size();
+  if (synthetic && have_unsent_events) {
+    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) {
   TRACE_EVENT0("input", "GestureEventQueue::ProcessGestureAck");
 
   if (coalesced_gesture_events_.empty()) {
     DLOG(ERROR) << "Received unexpected ACK for event type " << type;
     return;
   }
 
@@ skipping 180 matching lines @@
     return 0;
 
   if (!ignore_next_ack_)
     return 1;
 
   DCHECK_GT(coalesced_gesture_events_.size(), 1U);
   return 2;
 }
 
 }  // namespace content