code: @Override
    public void setDraggable(Element elem, String draggable) {
      super.setDraggable(elem, draggable);
      if ("true".equals(draggable)) {
        elem.getStyle().setProperty("webkitUserDrag", "element");
      } else {
        elem.getStyle().clearProperty("webkitUserDrag");
      }
    }
docs: Webkit based browsers require that we set the webkit-user-drag style
  attribute to make an element draggable.