Draggable.prototype.updateDrag = Draggable.prototype.updateDrag.wrap(function(callOriginal, event, pointer){
  callOriginal(event, pointer);
  Element.clonePosition('popup_bg', this.element);
})
Draggable.prototype.initialize = Draggable.prototype.initialize.wrap(function(callOriginal, element){
  var ret = callOriginal(element);
  var iframe = Element.insert(this.element, { after: new Element('iframe', {style: 'position:absolute;', id: "popup_bg", frameborder: "0", scrolling: "no"}) });
  iframe.style.width = this.element.getWidth()+'px';
  iframe.style.height = this.element.getHeight()+'px';
  return ret;
})