code: func (b *Box) SetDrawFunc(handler func(screen tcell.Screen, x, y, width, height int) (int, int, int, int)) *Box {
  	b.draw = handler
  	return b
  }
docs: // SetDrawFunc sets a callback function which is invoked after the box primitive
  // has been drawn. This allows you to add a more individual style to the box
  // (and all primitives which extend it).
  //
  // The function is provided with the box's dimensions (set via SetRect()). It
  // must return the box's inner dimensions (x, y, width, height) which will be
  // returned by GetInnerRect(), used by descendent primitives to draw their own
  // content.