Make pop-up menus more easily dismissable.

remoting/webapp/menu_button.css

 /* Copyright (c) 2012 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.
  */
 
 .menu-button {
   position: relative;
 }
 
 .menu-button-activator + ul {
@@ skipping 31 matching lines @@
 }
 
 .menu-button ul {
   padding: 0;
   margin: 0;
   list-style-type: none;
   background: white;
   outline: 1px solid rgba(0, 0, 0, 0.2);
   padding: 0 0 6px;
   box-shadow: 0px 2px 4px rgba(0, 0, 0, 0.2);
-  z-index: 1;
+  z-index: 2;
 }
 
 .menu-button li {
   padding: 6px 44px 6px 30px;
   white-space: nowrap;
 }
 
 .menu-button li:hover {
   background-color: #EEE;
 }
@@ skipping 18 matching lines @@
   -webkit-transition: opacity 0.218s;
 }
 
 .help-icon:hover, .help-icon.active {
   opacity: 0.5;
 }
 
 #help-feedback-toolbar {
   margin-left: 8px;
   margin-top: 4px;
+}
+
+.menu-button-click-trap {
+  position: fixed;
+  top: 0;
+  left: 0;
+  width: 100%;
+  height: 100%;
+  z-index: 1;
 }