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

[Eli Zaretskii <eliz@gnu.org>]

> Date: Tue, 29 Oct 2024 18:27:05 +0100
> From:  Eshel Yaron via "Bug reports for GNU Emacs,
>  the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>
> 
> 
> 
> The new implementation of string-pixel-width has some unexpected effect
> when it is called from mode-line-format, as happens for example when
> mode-line-format-right-align occurs in mode-line-format:
> 
> 1. emacs -Q
> 2. (setq-default mode-line-format
> 	      '("" (:eval (progn (string-pixel-width "foo") nil))))
> 3. C-x C-f /path/to/emacs/lisp/subr.el
> 4. C-SPC
> 5. C-n
> 
> At this point the region is expected to be active since we activated it
> in step 4.  But in step 5 the mode line is updated, which calls
> string-pixel-width, which in turn unexpectedly disables the region.

Thanks, should be fixed now.

> I'm not really sure why this happens...

It happens because string-pixel-width modifies a buffer, and that sets
deactivate-mark, which then causes the region to be deactivated when
a command finishes.  When you inject string-pixel-width into
mode-line-format, you indirectly cause it to be called from C-n and
the like, because those evaluate the mode-line format.  So doing that
is quite a risky thing, in general.