Implements frame sniffer to watch iframe loading state.

chrome/browser/resources/chromeos/login/oobe.js

 // Copyright (c) 2011 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.
 
 /**
  * @fileoverview Out of the box experience flow (OOBE).
  * This is the main code for the OOBE WebUI implementation.
  */
 
 var localStrings = new LocalStrings();
@@ skipping 118 matching lines @@
         }
       } else {
         // Start fading animation for login display.
         oldStep.classList.add('faded');
         newStep.classList.remove('faded');
       }
 
       // Adjust inner container height based on new step's height.
       $('inner-container').style.height = newStep.offsetHeight + 'px';
 
-      if (this.currentStep_ != nextStepIndex) {
+      if (this.currentStep_ != nextStepIndex &&
+          !oldStep.classList.contains('hidden')) {
         oldStep.addEventListener('webkitTransitionEnd', function f(e) {
           oldStep.removeEventListener('webkitTransitionEnd', f);
           oldStep.classList.add('hidden');
         });
       } else {
         // First screen on OOBE launch.
         newHeader.classList.remove('right');
       }
       this.currentStep_ = nextStepIndex;
       $('oobe').className = nextStepId;
@@ skipping 67 matching lines @@
         this.appendButtons_(screen.buttons);
       }
     },
 
     /**
      * Prepares screens to use in login display.
      */
     prepareForLoginDisplay_ : function() {
       for (var i = 0, screenId; screenId = this.screens_[i]; ++i) {
         var screen = $(screenId);
-
         screen.classList.add('faded');
         screen.classList.remove('right');
         screen.classList.remove('left');
       }
     }
   };
 
   /**
    * Setups given "select" element using the list and adds callback.
    * @param {!Element} select Select object to be updated.
@@ skipping 336 matching lines @@
 document.onselectstart = function(e) {
   e.preventDefault();
 }
 
 // Disable dragging.
 document.ondragstart = function(e) {
   e.preventDefault();
 }
 
 document.addEventListener('DOMContentLoaded', cr.ui.Oobe.initialize);