bug#63988: 30.0.50; Recent header line format changes cause spin/seg fault with format-mode-line

[Eli Zaretskii <eliz@gnu.org>]

> From: Aaron Jensen <aaronjensen@gmail.com>
> Date: Sat, 10 Jun 2023 07:59:30 -0400
> Cc: 63988@debbugs.gnu.org
> 
> > > > So maybe we should declare this feature a failed experiment and remove
> > > > it?
> > >
> > > I'll admit I don't really understand the change. Is it actually
> > > evaluating the cdr of the eval form up to two additional times to in
> > > order to determine whether or not to display the headerline at all?
> >
> > Only two?
> 
> Well, 2 per render. I'm just guessing at reading the diff.

window_wants_header_line is called in many other places as well.

> I'd personally prefer performance to being able to hide the header
> line optionally

What's wrong with setting header-line-format to nil?

> though I still don't really understand the change and
> why the one time it's eval'd in order to get the actual text/format
> can't be used to determine whether or not to show the header line.

Mainly because the window layout decisions need to know whether there
will be a header line before it's actually displayed.