bug#63825: 29.0.90; The header line should be hidden when empty

[Spencer Baugh]

Eli Zaretskii <eliz@gnu.org> writes:
>> From: Eshel Yaron <me@eshelyaron.com>
>> Cc: Spencer Baugh <sbaugh@janestreet.com>,  63825@debbugs.gnu.org
>> Date: Thu, 01 Jun 2023 19:45:39 +0300
>> 
>> Eli Zaretskii <eliz@gnu.org> writes:
>> 
>> > header-line-format supports :eval and :when, so why cannot
>> > which-function-mode use those to yield nil when there's nothing to
>> > show?
>> 
>> I thought so as well, but it seems that the header line is displayed
>> even when the value of `header-line-format` yields nil, if it isn't
>> plain nil.
>> 
>> So this shows an empty header line:
>> 
>> --8<---------------cut here---------------start------------->8---
>> (setq-default header-line-format '(:eval nil))
>> --8<---------------cut here---------------end--------------->8---
>> 
>> (In Emacs master with -Q.)
>
> It shouldn't be hard to make this special case behave as you want.
> Specifically, if the header-line-format is just a single cons cell,
> and the car of that cons cell is either :eval or a symbol, and the
> result of evaluation those yields nil, don't display the header line.
> (I don't really like the idea of not displaying the mode line under
> the same conditions.)
>
> Patches welcome.

Ok, the easy way to achieve that is to run format-mode-line on the
header-line-format and if it evaluates to "", don't display the header
line.  That also ignores the cases where header-line-format is multiple
cons cells, all of which evaluate to nil, and other such scenarios.  Is
that an acceptable approach to you?