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

[Eli Zaretskii]

> From: Stefan Monnier <monnier@iro.umontreal.ca>
> Cc: yantar92@posteo.net,  64596@debbugs.gnu.org
> Date: Sat, 15 Jul 2023 14:15:41 -0400
> 
> > Or why redisplay_internal does this:
> >
> >   consider_all_windows_p = (update_mode_lines
> >                         || windows_or_buffers_changed);
> 
> Sorry, I skipped this part in my previous email.
> I understand the above code, but I'm not sure what explanation might
> be needed so you can understand it as well.  Maybe the problem is the
> name `consider_all_windows_p` which suggests that all windows will be
> updated, but it only says that we should loop through all the windows to
> try and find those which need to be updated.

The actual issue with the above is different: it implies that
update_mode_lines _always_ indicates that more than a single mode line
should be updated (which is why we need to "consider all windows").
But is that actually true, i.e. does every place that assigns non-zero
to update_mode_lines indeed perform a change which makes it
_necessary_ to consider non-selected windows?