bug#64596: 30.0.50; On FIXME: in src/buffer.c:1481 (force-mode-line-update)

[Ihor Radchenko <yantar92@posteo.net>]

Hello,

`force-mode-line-update' has the following FIXME:

  if (!NILP (all))
    {
      update_mode_lines = 10;
      /* FIXME: This can't be right.  */
      current_buffer->prevent_redisplay_optimizations_p = true;
    }
  else if (buffer_window_count (current_buffer))
    {
      bset_update_mode_line (current_buffer);
      current_buffer->prevent_redisplay_optimizations_p = true;
    }

This FIXME has been introduced in 655ab9a3800, shortly after
ecda65d4f7e, which moved this code from `set-buffer-modified-p'.

AFAIU, the purpose of disabling redisplay optimizations is avoiding the
situation when the modification flag is unset in the buffer, but the
buffer was actually modified, and has to be redrawn.

If my understanding is correct,
current_buffer->prevent_redisplay_optimizations_p = true does not belong
to `force-mode-line-update', but rather to `restore-buffer-modified-p'.

I also grepped through src/display.c looking at the usage of
update_mode_lines, and there seems to be no obvious situation where
update_mode_lines being non-nil is ignored.


In GNU Emacs 30.0.50 (build 1, x86_64-pc-linux-gnu, GTK+ Version
 3.24.38, cairo version 1.17.8) of 2023-07-06 built on localhost
Repository revision: d97b77e6c66db46b198c696f83458aa141794727
Repository branch: master
Windowing system distributor 'The X.Org Foundation', version 11.0.12101008
System Description: Gentoo Linux


-- 
Ihor Radchenko // yantar92,
Org mode contributor,
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