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

[Stefan Monnier via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

>> It causes infinite recursion, since format-mode-line also calls
>> window_wants_header_line (indirectly).

I wonder why `format-mode-line` calls `window_wants_header_line`.
Is there a deep technical reason, or just an accident of how we
currently implement the feature?

> However, I wonder whether we should rethink this minor feature.
> Perhaps this minor convenience is not worth the complications?

IIUC This is talking about commit
4f66cbbfe520ee31ef26676e09a926217d9736fe which extends the special
treatment of a nil `header-line-format` to also hide the header line
when its value is "equivalent" to nil.

In the past I've wished for a non-nil mode-line that is not displayed
because it's equivalent to nil, so I think the feature makes sense, but
I agree it's not super important.  Maybe if we want to make it work
well, the better solution is to memoize the computation of
(format-mode-line header-line-format) so that it's called at most once
per redisplay?


        Stefan