end
function update_metadata(pane)
  local y = 0
  pane.line_cache = {}
  for i=1,#pane.lines do
    local line = pane.lines[i]
    pane.line_cache[i] = {}
    line.start_relative_y = y
    if line.mode == 'text' then
      -- semi-permanently initialize some cached stuff until the next edit
      Text.compute_fragments(pane, i)
      Text.populate_screen_line_starting_pos(pane, i)
      y = y + Line_height*#pane.line_cache[i].screen_line_starting_pos
    elseif line.mode == 'drawing' then
      -- nothing
      y = y + Drawing.pixels(line.h, Display_settings.column_width) + Drawing_padding_height
    else
      print(line.mode)
      assert(false)
    end
  end
  pane.height = y

    update_metadata(pane)