code: def updown(self, increment):
          
          scroll_line_num = self.screen_height - 4
          # paging
          if increment == -1 and self.markline == 0 and self.topline != 0:
              self.topline -= 1
          elif increment == 1 and self.markline + self.topline != len(self._lines) - 1 and self.markline == scroll_line_num:
              self.topline += 1
          # scroll
          if increment == -1 and self.markline != 0:
              self.markline -= 1
          elif increment == 1 and self.markline != scroll_line_num and self.markline < len(self._lines) - 1:
              self.markline += 1
docs: 屏幕上下滚动
  
          :params incrment: 1 向下滚动
                            -1 向上滚动