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

[Aaron Jensen <aaronjensen@gmail.com>]

On Sat, Jun 10, 2023 at 9:04 AM Eli Zaretskii <eliz@gnu.org> wrote:
>
> > 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?

What I meant is that I have no use for this feature and that I'd
rather be required to set it to nil to hide it. That's me personally
though, I don't know the impetus for the original addition.

> > 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.

Ah, that makes sense.