Non passive touch end or touch cancel listeners should not block scroll.

ui/events/blink/input_handler_proxy.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 "ui/events/blink/input_handler_proxy.h"
 
 #include <stddef.h>
 
 #include <algorithm>
 
@@ skipping 825 matching lines @@
     }
   }
 
   // Merge |touch_start_result_| and |result| so the result has the highest
   // priority value according to the sequence; (DROP_EVENT,
   // DID_HANDLE_NON_BLOCKING, DID_NOT_HANDLE).
   if (touch_start_result_ == kEventDispositionUndefined ||
       touch_start_result_ == DROP_EVENT || result == DID_NOT_HANDLE)
     touch_start_result_ = result;
 
+  // If |result| is still DROP_EVENT look at the touch end as

[tdresser, 2016/04/13 16:52:35]

look at the touch end handler

[dtapuska, 2016/04/13 18:00:15]

Done.

+  // we may not want to discard the entire touch sequence. Note this
+  // code is explicitly after the assignment of the |touch_start_result_|
+  // so the touch moves are not sent to the main thread un-necessarily.
+  if (result == DROP_EVENT &&
+      input_handler_->GetEventListenerProperties(
+          cc::EventListenerClass::kTouchEndOrCancel) !=
+          cc::EventListenerProperties::kNone) {
+    result = DID_HANDLE_NON_BLOCKING;
+  }
+
   return result;
 }
 
 InputHandlerProxy::EventDisposition InputHandlerProxy::HandleTouchMove(
     const blink::WebTouchEvent& touch_event) {
   if (touch_start_result_ != kEventDispositionUndefined)
     return static_cast<EventDisposition>(touch_start_result_);
   return DID_NOT_HANDLE;
 }
 
@@ skipping 489 matching lines @@
   // is made asynchronously, to minimize divergence between main thread and
   // impl thread event handling paths.
   base::ThreadTaskRunnerHandle::Get()->PostTask(
       FROM_HERE,
       base::Bind(&InputScrollElasticityController::ObserveGestureEventAndResult,
                  scroll_elasticity_controller_->GetWeakPtr(), gesture_event,
                  scroll_result));
 }
 
 }  // namespace ui