function Text.move_cursor_down_to_next_text_line_while_scrolling_again_if_necessary()
  while Cursor_line <= #Lines do
    if Lines[Cursor_line].mode == 'text' then
      break
    end
    Cursor_line = Cursor_line + 1
  end
  -- hack: insert a text line at bottom of file if necessary
  if Cursor_line > #Lines then
    assert(Cursor_line == #Lines+1)
    table.insert(Lines, {mode='text', data=''})
  end
  if Cursor_line > Screen_bottom_line then
    Screen_top_line = Cursor_line
    Text.scroll_up_while_cursor_on_screen()
  end
end
function Text.scroll_up_while_cursor_on_screen()
  local y = Screen_height - 15*Zoom -- for Cursor_line
  while true do
    if Screen_top_line == 1 then break end
    y = y - 15*Zoom
    if Lines[Screen_top_line].mode == 'drawing' then
      y = y - Drawing.pixels(Lines[Screen_top_line].h)
    end
    if y < 15*Zoom then
      break
    end
    Screen_top_line = Screen_top_line - 1
  end
end

    Text.move_cursor_down_to_next_text_line_while_scrolling_again_if_necessary()

      if Lines[Screen_top_line].mode == 'text' then
        y = y - 15*Zoom
      else

      y = y - 15*Zoom
      if Lines[Screen_top_line].mode == 'drawing' then

    Text.move_cursor_down_to_next_text_line_while_scrolling_again_if_necessary()

* The text cursor will always stay on the screen. This can have some strange
  implications:
    * A long series of drawings will get silently skipped when you hit
      page-down, until a line of text can be showed on screen.
    * If there's no line of text at the bottom of the file, one will be
      created.
  So far this app isn't really designed for all-drawing files. I'm really just
  targeting mostly-text files with a few drawings mixed in.