Trackpad scrolling could lag on MacOSX

content/browser/renderer_host/input/gesture_event_queue_unittest.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 <stddef.h>
 
 #include <memory>
 #include <utility>
@@ skipping 1138 matching lines @@
       WebInputEvent::GestureScrollUpdate,
       WebInputEvent::GestureScrollUpdate};
 
   for (unsigned i = 0; i < sizeof(expected) / sizeof(WebInputEvent::Type);
       i++) {
     WebGestureEvent merged_event = GestureEventQueueEventAt(i);
     EXPECT_EQ(expected[i], merged_event.type);
   }
 }
 
+TEST_F(GestureEventQueueTest, CoalescesSyntheticScrollBeginEndEvents) {
+  // Test coalescing of only GestureScrollBegin/End events.
+  SimulateGestureEvent(WebInputEvent::GestureScrollUpdate,
+                       blink::WebGestureDeviceTouchpad);
+  EXPECT_EQ(1U, GetAndResetSentGestureEventCount());
+  EXPECT_EQ(1U, GestureEventQueueSize());
+
+  WebGestureEvent synthetic_end = SyntheticWebGestureEventBuilder::Build(
+      WebInputEvent::GestureScrollEnd, blink::WebGestureDeviceTouchpad);
+  synthetic_end.data.scrollEnd.synthetic = true;
+
+  SimulateGestureEvent(synthetic_end);
+  EXPECT_EQ(0U, GetAndResetSentGestureEventCount());
+  EXPECT_EQ(2U, GestureEventQueueSize());
+
+  // Synthetic begin will remove the unsent synthetic end.
+  WebGestureEvent synthetic_begin = SyntheticWebGestureEventBuilder::Build(
+      WebInputEvent::GestureScrollBegin, blink::WebGestureDeviceTouchpad);
+  synthetic_begin.data.scrollBegin.synthetic = true;
+
+  SimulateGestureEvent(synthetic_begin);
+  EXPECT_EQ(0U, GetAndResetSentGestureEventCount());
+  EXPECT_EQ(1U, GestureEventQueueSize());
+}
+
 }  // namespace content