Use offsetX and offsetY to position the fake mouse cursor.

remoting/webapp/crd/js/desktop_connected_view.js

Index: remoting/webapp/crd/js/desktop_connected_view.js
diff --git a/remoting/webapp/crd/js/desktop_connected_view.js b/remoting/webapp/crd/js/desktop_connected_view.js
index e4ce2acb0dab98ad18634aca4358d2bf88b58172..0a14b08e267adaeef50a464dc50bbd6841ac3f63 100644
--- a/remoting/webapp/crd/js/desktop_connected_view.js
+++ b/remoting/webapp/crd/js/desktop_connected_view.js
@@ -79,8 +79,8 @@ remoting.DesktopConnectedView = function(session, container, host, mode,
   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 */