def jump_to_line l
    l = l.clamp 0, lines - 1
    return if @topline == l
    @topline = l
    @botline = [l + buffer.content_height, lines].min
    buffer.mark_dirty
  end
def jump_to_line l
    l = l.clamp 0, lines - 1
    return if @topline == l
    @topline = l
    @botline = [l + buffer.content_height, lines].min
    buffer.mark_dirty
  end
set top line to l