Changes PlatformWindowMus to ack event callback before dispatching

ui/views/mus/platform_window_mus.cc

 // Copyright 2015 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/views/mus/platform_window_mus.h"
 
 #include "build/build_config.h"
 #include "components/mus/public/cpp/property_type_converters.h"
 #include "components/mus/public/cpp/window_property.h"
 #include "components/mus/public/interfaces/window_manager.mojom.h"
@@ skipping 194 matching lines @@
 }
 
 void PlatformWindowMus::OnRequestClose(mus::Window* window) {
   delegate_->OnCloseRequest();
 }
 
 void PlatformWindowMus::OnWindowInputEvent(
     mus::Window* view,
     mus::mojom::EventPtr event,
     scoped_ptr<base::Closure>* ack_callback) {
+  // It's possible dispatching the event will spin a nested message loop. Ack
+  // the callback now, otherwise we appear unresponsive for the life of the
+  // nested message loop.
+  (*ack_callback)->Run();
+  ack_callback->reset();
   scoped_ptr<ui::Event> ui_event(event.To<scoped_ptr<ui::Event>>());
   delegate_->DispatchEvent(ui_event.get());
 }
 
 }  // namespace views