code: def center_visible_line(self, before_scroll_offset=False,
                              after_scroll_offset=False):
          
          return (self.first_visible_line(after_scroll_offset) +
                  (self.last_visible_line(before_scroll_offset) -
                   self.first_visible_line(after_scroll_offset)) // 2
                 )
docs: Like `first_visible_line`, but for the center visible line.