code: def _add_or_remove_scrollbar(self, width, height, dy):
          
          if self._scroll_bar is None and len(self._options) > height:
              self._scroll_bar = _ScrollBar(
                  self._frame.canvas, self._frame.palette, self._x + width - 1, self._y + dy,
                  height, self._get_pos, self._set_pos)
          elif self._scroll_bar is not None and len(self._options) <= height:
              self._scroll_bar = None
docs: Add or remove a scrollbar from this listbox based on height and available options.
  
          :param width: Width of the Listbox
          :param height: Height of the Listbox.
          :param dy: Vertical offset from top of widget.