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

[Stefan Monnier]

> 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.

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

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.


        Stefan