Position the <img> cursor relative to the client plugin, not the window.

remoting/webapp/base/html/main.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.
  */
 
 html, body, div, span, applet, object, iframe, h1, h2, h3, h4, h5, h6, p,
 blockquote, pre, a, abbr, acronym, address, big, cite, code, del, dfn, em,
 font, img, ins, kbd, q, s, samp, small, strike, strong, sub, sup, tt, var, dl,
 dt, dd, ol, ul, li, fieldset, form, label, legend, table, caption, tbody,
 tfoot, thead, tr, th, td, button {
@@ skipping 715 matching lines @@
 
 .debug-region-rect {
   position: absolute;
   background-color: rgba(255, 0, 255, 0.5);
   border-color: #000000;
   border-style: solid;
   border-width: 2px;
   box-sizing: border-box;
 }
 
+#client-container {
+  /* The mouse-cursor-overlay is positioned relative to the client plugin,
+   * which is contained within this element. */
+  position: relative;
+}
+
 .mouse-cursor-overlay {
   position: absolute;
   pointer-events: none;
 }