createSelectionBox = function() {
    var $this = $(this),
        data = $this.data(pluginName);

    data.selectionGlass = $(
      '<div />', {'class': pluginName + '-glass'}).css({
      'position': 'absolute',
      'top': 0,
      'left': 0,
      'height': this.scrollHeight,
      'width': this.scrollWidth,
      'overflow': 'hidden'}).appendTo($this);
    data.selectionBox = $(
      '<div />', {'class': pluginName + '-box'}).css({
      'position': 'absolute'
      }).appendTo(data.selectionGlass);
  }
Creates the selection box and the glass panel that isolates selectable
elements from the selection gesture events (required to prevent Flash
grabbing mouseup events for selections ending on top of Flash embeds).