GNU bug report logs - #74091
31.0.50; string-pixel-width in mode line disables region

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From: help-debbugs <at> gnu.org (GNU bug Tracking System)
To: Eli Zaretskii <eliz <at> gnu.org>
Cc: tracker <at> debbugs.gnu.org
Subject: bug#74091: closed (31.0.50; string-pixel-width in mode line
 disables region)
Date: Wed, 30 Oct 2024 16:02:02 +0000

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Your message dated Wed, 30 Oct 2024 18:01:11 +0200
with message-id <86ldy54m2g.fsf <at> gnu.org>
and subject line Re: bug#74091: 31.0.50; string-pixel-width in mode line disables region
has caused the debbugs.gnu.org bug report #74091,
regarding 31.0.50; string-pixel-width in mode line disables region
to be marked as done.

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From: Eshel Yaron <me <at> eshelyaron.com>
To: bug-gnu-emacs <at> gnu.org
Subject: 31.0.50; string-pixel-width in mode line disables region
Date: Tue, 29 Oct 2024 18:27:05 +0100

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.

I'm not really sure why this happens...  Also, if I now trace
string-pixel-width, then it no longer disables the region and everything
starts working as expected.


Thanks,

Eshel

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From: Eli Zaretskii <eliz <at> gnu.org>
To: Eshel Yaron <me <at> eshelyaron.com>
Cc: 74091-done <at> debbugs.gnu.org
Subject: Re: bug#74091: 31.0.50; string-pixel-width in mode line disables
 region
Date: Wed, 30 Oct 2024 18:01:11 +0200

> From: Eshel Yaron <me <at> eshelyaron.com>
> Cc: 74091 <at> debbugs.gnu.org
> Date: Wed, 30 Oct 2024 16:26:38 +0100
> 
> >> 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.
> 
> Great!  That seems to work.

Thanks for testing, I will close this bug.

> >> 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.
> 
> Hmm but string-pixel-width used to modify a buffer also in the old
> implementation, and that never caused this issue...

The new implementation also didn't cause this issue in some buffers.
For example, in *scratch*.  Trying to understand the logic of a bug is
never a good investment of time.

> And in both the old
> implementation and in the new one, the modification is in a different
> buffer, is that expected to disable the mark in the original buffer?

The variable deactivate-mark only becomes buffer-local if set;
otherwise the global value will be changed.

> > 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.
> 
> Well, that's how Emacs implements mode-line-format-right-align :)

One reason why I don't like it.  mode-line-format is evaluated in many
more contexts than most people realize, so putting arbitrary calls
there without a good understanding what those calls do and how is not
the best idea, although it will mostly work.

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