Drag and drop cleans up touch sequences from the source window.

ui/aura/window_event_dispatcher.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 "ui/aura/window_event_dispatcher.h"
 
 #include <stddef.h>
 
 #include "base/bind.h"
 #include "base/logging.h"
@@ skipping 510 matching lines @@
 
 ////////////////////////////////////////////////////////////////////////////////
 // WindowEventDispatcher, ui::GestureEventHelper implementation:
 
 bool WindowEventDispatcher::CanDispatchToConsumer(
     ui::GestureConsumer* consumer) {
   Window* consumer_window = ConsumerToWindow(consumer);
   return (consumer_window && consumer_window->GetRootWindow() == window());
 }
 
-void WindowEventDispatcher::DispatchCancelTouchEvent(
-    ui::GestureConsumer* raw_input_consumer,
-    ui::TouchEvent* event) {
-  // The touchcancel event's location is based on the last known location of
+void WindowEventDispatcher::DispatchSyntheticTouchEvent(ui::TouchEvent* event) {
+  // The synthetic event's location is based on the last known location of
   // the pointer, in dips. OnEventFromSource expects events with co-ordinates
   // in raw pixels, so we convert back to raw pixels here.

[sadrul, 2016/05/11 16:12:17]

Can you add a DCHECK() here that |event->type()| is always |ET_TOUCH_CANCELED|?
If that assumption changes, it'd be good to explicitly verify if we want to do
that.

[tdresser, 2016/05/11 16:32:31]

Done.

   event->UpdateForRootTransform(host_->GetRootTransform());
   DispatchDetails details = OnEventFromSource(event);
   if (details.dispatcher_destroyed)
     return;
 }
 
 ////////////////////////////////////////////////////////////////////////////////
 // WindowEventDispatcher, WindowObserver implementation:
 
 void WindowEventDispatcher::OnWindowDestroying(Window* window) {
@@ skipping 362 matching lines @@
   }
 
   // This flag is set depending on the gestures recognized in the call above,
   // and needs to propagate with the forwarded event.
   event->set_may_cause_scrolling(orig_event.may_cause_scrolling());
 
   return PreDispatchLocatedEvent(target, event);
 }
 
 }  // namespace aura