bug#74091: 31.0.50; string-pixel-width in mode line disables region

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

Eli Zaretskii <eliz@gnu.org> writes:

>> From: Eshel Yaron <me@eshelyaron.com>
>> Cc: 74091@debbugs.gnu.org
>> Date: Wed, 06 Nov 2024 09:01:16 +0100
>> 
>> FYI after spending a bit more time on this issue, I concluded that
>> (kill-all-local-variables t) is inherently problematic: it breaks
>> assumptions that Emacs relies on.  (See bug#73005 for another example.)
>> 
>> It doesn't seem like killing permanent-local variables in the work
>> buffers is necessary ATM, so the fix I'm using is the following:
>
> Can't _any_ variable become permanent-local, by virtue of some Lisp
> program or the user giving it a permanent-local property?
>
> More generally, how do we know that there are no permanent-local
> variables out there that affect layout, and thus affect the results of
> this function?
>
> I believe these considerations were those which led the author to use
> kill-all-local-variables like this: we want to be sure that nothing
> will dupe string-pixel-width into producing unexpected results.

Yes, this reasoning is perfectly understandable.  However, the current
way of killing all permanent-local variables has bad consequences that
that author, naturally, did not expect.  So my suggestion is to avoid
(kill-all-local-variables t) here and elsewhere.

For string-pixel-width specifically, we take care to clear variables
that may affect width calculation.  If other variables may interfere, we
should just clear those too.


Cheers,

Eshel