Support transitioning between platform scrolling and JS pinching

content/browser/renderer_host/input/touch_event_queue.cc

 // Copyright 2013 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/touch_event_queue.h"
 
 #include "base/auto_reset.h"
 #include "base/debug/trace_event.h"
 #include "base/stl_util.h"
 #include "content/browser/renderer_host/input/timeout_monitor.h"
@@ skipping 502 matching lines @@
     } else {
       scoped_ptr<TouchEventWithLatencyInfo> async_move =
           pending_async_touchmove_.Pass();
       async_move->event.cancelable = false;
       touch_queue_.push_front(new CoalescedWebTouchEvent(*async_move, true));
       SendTouchEventImmediately(*async_move);
       return;
     }
   }
 
-  // Note: Marking touchstart events as not-cancelable prevents them from
-  // blocking subsequent gestures, but it may not be the best long term solution
-  // for tracking touch point dispatch.
-  if (send_touch_events_async_)
+  // Note: Touchstart events are marked cancelable to allow transitions between
+  // platform scrolling and JS pinching. Touchend events, however, remain
+  // uncancelable, mitigating the risk of jank when transitioning to a fling.
+  if (send_touch_events_async_ && touch.event.type != WebInputEvent::TouchStart)
     touch.event.cancelable = false;
 
   // A synchronous ack will reset |dispatching_touch_|, in which case
   // the touch timeout should not be started.
   base::AutoReset<bool> dispatching_touch(&dispatching_touch_, true);
   SendTouchEventImmediately(touch);
   if (dispatching_touch_ && timeout_handler_)
     timeout_handler_->StartIfNecessary(touch);
 }
 
@@ skipping 217 matching lines @@
   if (event.type == WebInputEvent::TouchEnd ||
       event.type == WebInputEvent::TouchCancel) {
     // The points have been released. Erase the ACK states.
     for (unsigned i = 0; i < event.touchesLength; ++i) {
       const WebTouchPoint& point = event.touches[i];
       if (point.state == WebTouchPoint::StateReleased ||
           point.state == WebTouchPoint::StateCancelled)
         touch_consumer_states_.clear_bit(point.id);
     }
   } else if (event.type == WebInputEvent::TouchStart) {
+    if (touch_scrolling_mode_ == TOUCH_SCROLLING_MODE_ASYNC_TOUCHMOVE &&
+        ack_result == INPUT_EVENT_ACK_STATE_CONSUMED) {
+      send_touch_events_async_ = false;
+    }
     for (unsigned i = 0; i < event.touchesLength; ++i) {
       const WebTouchPoint& point = event.touches[i];
       if (point.state == WebTouchPoint::StatePressed) {
         if (ack_result != INPUT_EVENT_ACK_STATE_NO_CONSUMER_EXISTS)
           touch_consumer_states_.mark_bit(point.id);
         else
           touch_consumer_states_.clear_bit(point.id);
       }
     }
   }
 }
 
 }  // namespace content