GNU bug report logs - #63825
29.0.90; The header line should be hidden when empty

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From: Spencer Baugh <sbaugh <at> janestreet.com>
To: Eshel Yaron <me <at> eshelyaron.com>
Cc: Eli Zaretskii <eliz <at> gnu.org>, 63825 <at> debbugs.gnu.org
Subject: bug#63825: 29.0.90; The header line should be hidden when empty
Date: Thu, 01 Jun 2023 13:37:18 -0400

Eshel Yaron <me <at> eshelyaron.com> writes:
> Eli Zaretskii <eliz <at> 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:
>
> (setq-default header-line-format '(:eval nil))
>
> (In Emacs master with -Q.)

Yes, this is what I meant.  Sorry, my example was not chosen carefully.
I should have chosen an example which yielded an actually empty string
when processed by format-mode-line, such as '(:eval nil);
(format-mode-line '(:eval nil)) evaluates to "", which I think should
cause the header-line to not appear.

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