Use offsetX and offsetY to position the fake mouse cursor.

remoting/webapp/crd/js/desktop_connected_view.js

 // 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.
 
 /**
  * @fileoverview
  * Class handling user-facing aspects of the client session.
  */
 
 'use strict';
@@ skipping 61 matching lines @@
   this.notifyClientResolutionTimer_ = null;
 
   /** @type {Element} @private */
   this.mouseCursorOverlay_ =
       this.container_.querySelector('.mouse-cursor-overlay');
 
   /** @type {Element} */
   var img = this.mouseCursorOverlay_;
   /** @param {Event} event @private */
   this.updateMouseCursorPosition_ = function(event) {
-    img.style.top = event.y + 'px';
-    img.style.left = event.x + 'px';
+    img.style.top = event.offsetY + 'px';
+    img.style.left = event.offsetX + 'px';
   };
 
   /** @type {number?} @private */
   this.bumpScrollTimer_ = null;
 
   // Bump-scroll test variables. Override to use a fake value for the width
   // and height of the client plugin so that bump-scrolling can be tested
   // without relying on the actual size of the host desktop.
   /** @type {number} @private */
   this.pluginWidthForBumpScrollTesting_ = 0;
@@ skipping 952 matching lines @@
  * @param {Object} message The parsed extension message data.
  * @return {boolean} True if the message was recognized, false otherwise.
  */
 remoting.DesktopConnectedView.prototype.handleExtensionMessage =
     function(type, message) {
   if (this.videoFrameRecorder_) {
     return this.videoFrameRecorder_.handleMessage(type, message);
   }
   return false;
 };