void addWindowSubclass() {
   long /*int*/ hwndChild = OS.GetWindow(browser.handle, OS.GW_CHILD);
   if (SubclassProc == null) {
     SubclassProc = new Callback(MozillaDelegate.class, "windowProc", 4); // $NON-NLS-1$
     MozillaProc = OS.GetWindowLongPtr(hwndChild, OS.GWL_WNDPROC);
   }
   OS.SetWindowLongPtr(hwndChild, OS.GWL_WNDPROC, SubclassProc.getAddress());
 }