-- text to render:
  --   mapping from pane id to arrays of lines as in lines.love

  -- some references shared by panes backed by the same file
  --   line contents when panes are not editable (editable panes can diverge)
  --   links to neighboring panes (never in Surface, can never diverge between panes)

  result.line_cache = deepcopy(Cache[id].line_cache)

  result.line_cache = deepcopy(Cache[id].line_cache)  -- should be tiny; deepcopy is just to eliminate any chance of aliasing