Prevent delivery of tab release key events while alt tabbing.

ash/wm/window_cycle_controller.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 "ash/wm/window_cycle_controller.h"
 
 #include "ash/metrics/user_metrics_recorder.h"
 #include "ash/session/session_state_delegate.h"
 #include "ash/shell.h"
 #include "ash/wm/mru_window_tracker.h"
@@ skipping 29 matching lines @@
 
 WindowCycleEventFilter::WindowCycleEventFilter() {
   Shell::GetInstance()->AddPreTargetHandler(this);
 }
 
 WindowCycleEventFilter::~WindowCycleEventFilter() {
   Shell::GetInstance()->RemovePreTargetHandler(this);
 }
 
 void WindowCycleEventFilter::OnKeyEvent(ui::KeyEvent* event) {
+  // Until the alt key is released, all key events are handled by this window
+  // cycle controller: https://crbug.com/340339.
+  event->StopPropagation();
   // Views uses VKEY_MENU for both left and right Alt keys.
   if (event->key_code() == ui::VKEY_MENU &&
       event->type() == ui::ET_KEY_RELEASED) {
     Shell::GetInstance()->window_cycle_controller()->StopCycling();
     // Warning: |this| will be deleted from here on.
   }
 }
 
 }  // namespace
 
@@ skipping 59 matching lines @@
       active_window_before_window_cycle_ != active_window_after_window_cycle) {
     Shell::GetInstance()
         ->metrics()
         ->task_switch_metrics_recorder()
         .OnTaskSwitch(TaskSwitchMetricsRecorder::WINDOW_CYCLE_CONTROLLER);
   }
   active_window_before_window_cycle_ = nullptr;
 }
 
 }  // namespace ash