GNU bug report logs - #78953
31.0.50; tab-bar-format-align-right alignment breaks if field widths change

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Message #5 received at submit <at> debbugs.gnu.org (full text, mbox):

From: Daniel Mendler <mail <at> daniel-mendler.de>
To: bug-gnu-emacs <at> gnu.org
Subject: 31.0.50; tab-bar-format-align-right alignment breaks if field
 widths change
Date: Fri, 04 Jul 2025 08:39:24 +0200

When fields in `tab-bar-format' after `tab-bar-format-align-right'
change their width between two calls, then the alignment breaks - the
right aligned tab field jumps around. Example configuration which uses a
`global-mode-string' with a random width:

(setq tab-bar-format
  '( tab-bar-format-tabs
     tab-bar-format-align-right
     tab-bar-format-global))

(setq global-mode-string
      '("|" (:eval (make-string (random 10) ?#)) "|"))

(run-at-time 1 1 #'force-mode-line-update t)

A more realistic configuration with a timer (and a variable pitch face)
also shows the problem. A possible solution could be to handle
`tab-bar-format-align-right' in `tab-bar-format-list', such that the
functions in the `tab-bar-format' list after
`tab-bar-format-align-right' are not called twice, such that the
computed field width matches the displayed field.

In GNU Emacs 31.0.50 (build 1, x86_64-pc-linux-gnu, GTK+ Version
 3.24.49, cairo version 1.18.4) of 2025-06-20
Windowing system distributor 'The X.Org Foundation', version 11.0.12101016
System Description: Debian GNU/Linux 13 (trixie)

Configured using:
 'configure --prefix=$HOME/.local/share/emacs
 --without-compress-install --with-tree-sitter --with-native-compilation
 --with-dbus --without-selinux --without-threads --disable-gc-mark-trace
 --without-gsettings --without-gpm --with-cairo --with-cairo-xcb
 --with-xinput2 --with-x-toolkit=gtk3 --without-toolkit-scroll-bars'

Configured features:
CAIRO DBUS FREETYPE GIF GLIB GMP GNUTLS HARFBUZZ JPEG LIBOTF LIBSYSTEMD
LIBXML2 MODULES NATIVE_COMP NOTIFY INOTIFY PDUMPER PNG RSVG SECCOMP
SOUND SQLITE3 TIFF TREE_SITTER WEBP X11 XDBE XIM XINERAMA XINPUT2 XPM
XRANDR GTK3 ZLIB

Message #8 received at 78953 <at> debbugs.gnu.org (full text, mbox):

From: Juri Linkov <juri <at> linkov.net>
To: Daniel Mendler <mail <at> daniel-mendler.de>
Cc: 78953 <at> debbugs.gnu.org
Subject: Re: bug#78953: 31.0.50; tab-bar-format-align-right alignment breaks
 if field widths change
Date: Fri, 04 Jul 2025 09:52:58 +0300

> When fields in `tab-bar-format' after `tab-bar-format-align-right'
> change their width between two calls, then the alignment breaks - the
> right aligned tab field jumps around. Example configuration which uses a
> `global-mode-string' with a random width:
>
> (setq tab-bar-format
>   '( tab-bar-format-tabs
>      tab-bar-format-align-right
>      tab-bar-format-global))
>
> (setq global-mode-string
>       '("|" (:eval (make-string (random 10) ?#)) "|"))
>
> (run-at-time 1 1 #'force-mode-line-update t)

Thanks, I confirm the problem.

> A more realistic configuration with a timer (and a variable pitch face)
> also shows the problem. A possible solution could be to handle
> `tab-bar-format-align-right' in `tab-bar-format-list', such that the
> functions in the `tab-bar-format' list after
> `tab-bar-format-align-right' are not called twice, such that the
> computed field width matches the displayed field.

So need to update :align-to in tab-bar-format-list.

Message #11 received at 78953 <at> debbugs.gnu.org (full text, mbox):

From: Daniel Mendler <mail <at> daniel-mendler.de>
To: Juri Linkov <juri <at> linkov.net>
Cc: 78953 <at> debbugs.gnu.org
Subject: Re: bug#78953: 31.0.50; tab-bar-format-align-right alignment breaks
 if field widths change
Date: Fri, 04 Jul 2025 09:21:38 +0200

Juri Linkov <juri <at> linkov.net> writes:

>> When fields in `tab-bar-format' after `tab-bar-format-align-right'
>> change their width between two calls, then the alignment breaks - the
>> right aligned tab field jumps around. Example configuration which uses a
>> `global-mode-string' with a random width:
>>
>> (setq tab-bar-format
>>   '( tab-bar-format-tabs
>>      tab-bar-format-align-right
>>      tab-bar-format-global))
>>
>> (setq global-mode-string
>>       '("|" (:eval (make-string (random 10) ?#)) "|"))
>>
>> (run-at-time 1 1 #'force-mode-line-update t)
>
> Thanks, I confirm the problem.

Thanks for checking.

>> A more realistic configuration with a timer (and a variable pitch face)
>> also shows the problem. A possible solution could be to handle
>> `tab-bar-format-align-right' in `tab-bar-format-list', such that the
>> functions in the `tab-bar-format' list after
>> `tab-bar-format-align-right' are not called twice, such that the
>> computed field width matches the displayed field.
>
> So need to update :align-to in tab-bar-format-list.

Yes, exactly.

Also another problem I've observed is that `string-pixel-width' is slow,
which is a problem with frequent updates/redisplays, for example when
scrolling a lot with `pixel-scroll-precision-mode'.

I suggest to update `:align-to' in `tab-bar-format-list' and also cache
the computed width and the formatted tab bar string. Then the cached
string can be compared with `equal-including-properties` and only if the
string changes, the pixel width needs to be recomputed.

Daniel

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