if line_index > Screen_top_line or pos >= Top_screen_line_starting_pos then

      if line_index > Screen_top_line or pos > Top_screen_line_starting_pos then

    -- previous text line
    local new_cursor_line = Cursor_line
    while new_cursor_line > 1 do
      new_cursor_line = new_cursor_line-1
      if Lines[new_cursor_line].mode == 'text' then
        local old_x = Text.cursor_x(Lines[new_cursor_line].data, Cursor_pos)
        Cursor_line = new_cursor_line
        Cursor_pos = Text.nearest_cursor_pos(Lines[Cursor_line].data, old_x)
        break

    if Top_screen_line_starting_pos == 1 then
      -- top line is done; skip to previous text line
      local new_cursor_line = Cursor_line
      while new_cursor_line > 1 do
        new_cursor_line = new_cursor_line-1
        if Lines[new_cursor_line].mode == 'text' then
          local old_x = Text.cursor_x(Lines[new_cursor_line].data, Cursor_pos)
          Cursor_line = new_cursor_line
          Cursor_pos = Text.nearest_cursor_pos(Lines[Cursor_line].data, old_x)
          break
        end
      end
      if Cursor_line < Screen_top_line then
        Screen_top_line = Cursor_line

    end
    if Cursor_line < Screen_top_line then
      Screen_top_line = Cursor_line

    else
      -- scroll up just one screen line in current line
      local screen_line_index = table.find(Lines[Cursor_line].screen_line_starting_pos, Top_screen_line_starting_pos)
      assert(screen_line_index > 1)
      Top_screen_line_starting_pos = Lines[Cursor_line].screen_line_starting_pos[screen_line_index-1]
      local s = string.sub(Lines[Cursor_line].data, Top_screen_line_starting_pos)
      Cursor_pos = Text.nearest_cursor_pos(s, Cursor_x)

scrolling:
  given moby dick, a file containing all text:
  page up moves top line on screen to bottom
  page down moves bottom line on screen to top
  cursor remains on screen
  cursor remains on text line
  'up' arrow with cursor at top of screen scrolls up one line (drawings still fully in or out)
    if top line wrapped before, it scrolls up by only one screen line
  'down' arrow with cursor at bottom of screen scrolls down one line (drawings still fully in or out)
    if top line wrapped before, it scrolls down by only one screen line