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: Sun, 16 Jul 2023 18:19:57 -0400
> 
> > That's not all of the uses of this flag.  Here's one example of other
> > uses:
> >
> >       if (some_windows
> >           && !f->redisplay
> >           && !w->redisplay
> >           && !XBUFFER (w->contents)->text->redisplay)
> >         continue;
> >
> > (This avoids updating the tool bar and the tab bar of the frame, and
> > there's a similar condition that avoids updating the frame's title.)
> 
> Right, but notice that what we're fundamentally checking is whether `w`
> (which is the frame's selected window here) needs to be updated, because
> that's the one currently "occupying" the tool-bar.

But testing the frame's redisplay flag will produce false positives,
because many changes that require to redraw the frame's windows don't
affect the tool bar or the frame's title.

> FWIW, I don't know why we don't also check
> `XBUFFER(w->contents)->redisplay` here.

We do:

          if (some_windows
              && !f->redisplay
              && !w->redisplay
              && !XBUFFER (w->contents)->text->redisplay)  <<<<<<<<<<<
            continue;

> Also, I still don't see why that makes you think:
> 
>     frame's redisplay flag is what causes us to consider a frame in the
>     first place.
> 
> Indeed, a frame's redisplay flag may be such a reason, but a window's
> flag or its text buffer's may be the reason instead.  A frame's
> `redisplay` flag does not need to be set for us to consider it.

No, my point is that fset_redisplay shouldn't set
windows_or_buffers_changed.