end
      if y > Screen_height then
        if line.screen_line_starting_pos then
          Bottom_screen_line_starting_pos = line.screen_line_starting_pos[#line.screen_line_starting_pos]
        else
          Bottom_screen_line_starting_pos = 1
        end

    -- next text line
    local new_cursor_line = Cursor_line
    while new_cursor_line < #Lines do
      new_cursor_line = new_cursor_line+1
      if Lines[new_cursor_line].mode == 'text' then
        Cursor_line = new_cursor_line
        Cursor_pos = Text.nearest_cursor_pos(Lines[Cursor_line].data, Cursor_x)
        break

    if Text.cursor_at_final_screen_line() then
      -- line is done, skip to next text line
      print('down: cursor at final screen line of its line')
      local new_cursor_line = Cursor_line
      while new_cursor_line < #Lines do
        new_cursor_line = new_cursor_line+1
        if Lines[new_cursor_line].mode == 'text' then
          Cursor_line = new_cursor_line
          Cursor_pos = Text.nearest_cursor_pos(Lines[Cursor_line].data, Cursor_x)
          print(Cursor_pos)
          break
        end
      end
      if Cursor_line > Screen_bottom_line then
        Screen_top_line = Cursor_line
        Text.scroll_up_while_cursor_on_screen()

    end
    if Cursor_line > Screen_bottom_line then
      Screen_top_line = Cursor_line

    else
      -- move down one screen line in current line
      print('cursor is NOT at final screen line of its line')
      local screen_line_index, screen_line_starting_pos = Text.pos_at_start_of_cursor_screen_line()
      new_screen_line_starting_pos = Lines[Cursor_line].screen_line_starting_pos[screen_line_index+1]
      print('switching pos of screen line at cursor from '..tostring(screen_line_starting_pos)..' to '..tostring(new_screen_line_starting_pos))
      local s = string.sub(Lines[Cursor_line].data, new_screen_line_starting_pos)
      Cursor_pos = new_screen_line_starting_pos + Text.nearest_cursor_pos(s, Cursor_x) - 1
      print('cursor pos is now '..tostring(Cursor_pos))
      Text.scroll_up_while_cursor_on_screen()

function Text.cursor_at_final_screen_line()
  if Lines[Cursor_line].screen_line_starting_pos == nil then
    return true
  end
  i=#Lines[Cursor_line].screen_line_starting_pos
  local spos = Lines[Cursor_line].screen_line_starting_pos[i]
  return spos <= Cursor_pos
end