Position.getWindowSize = function(w) {
  var width, height;
  w = w ? w : window;
  width = w.innerWidth || (w.document.documentElement.clientWidth || w.document.body.clientWidth);
  height = w.innerHeight || (w.document.documentElement.clientHeight || w.document.body.clientHeight);
 
  return { width: width, height: height };
}