bug#50721: Wrong point position or display while moving point

bug#50721: Wrong point position or display while moving point

[Cos Chan]

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From: rosettas@gmail.com
To: bug-gnu-emacs@gnu.org
Subject: 27.1; Wrong point position or display while moving point
--text follows this line--

This bug report will be sent to the Bug-GNU-Emacs mailing list
and the GNU bug tracker at debbugs.gnu.org.  Please check that
the From: line contains a valid email address.  After a delay of up
to one day, you should receive an acknowledgment at that address.

Please write in English if possible, as the Emacs maintainers
usually do not have translators for other languages.

Please describe exactly what actions triggered the bug, and
the precise symptoms of the bug.  If you can, give a recipe
starting from 'emacs -Q':

To change system tab width by
$ tabs 4
would create a problem that the display is wrong while moving point (for
example M-b/M-f/C-up/C-down or kill words/line).
The point or the characters would be in the wrong places until I manually
run M-x redraw-display to get it back.

I may reproduce this problem in Debian 10, 11 and FreeBSD 12, 13. Tested in
zsh and tcsh by add tabs 4 into .zshrc and .tcshrc

my .emacs file content:
(add-to-list 'load-path "~/.emacs.d/lisp/")
;; lisp scripts
(require 'sudo-save)
;; editting settings
(setq-default indent-tabs-mode t)
(setq-default tab-width 4)
(normal-erase-is-backspace-mode 1)
;; backup and autosave to temporary directory
(setq backup-directory-alist
      `((".*" . ,temporary-file-directory)))
(setq auto-save-file-name-transforms
      `((".*" ,temporary-file-directory t)))
;; set color to be compatible with dark background
(setq frame-background-mode 'dark)

If Emacs crashed, and you have the Emacs process in the gdb debugger,
please include the output from the following gdb commands:
    'bt full' and 'xbacktrace'.
For information about debugging Emacs, please read the file
/usr/share/emacs/27.1/etc/DEBUG.

In GNU Emacs 27.1 (build 1, x86_64-pc-linux-gnu)
 of 2021-03-27, modified by Debian built on x86-conova-01
System Description: Debian GNU/Linux 11 (bullseye)



In GNU Emacs 27.1 (build 1, x86_64-pc-linux-gnu)
 of 2021-03-27, modified by Debian built on x86-conova-01
System Description: Debian GNU/Linux 11 (bullseye)

Recent messages:
Loading /etc/emacs/site-start.d/00debian.el (source)...done
Loading /etc/emacs/site-start.d/50tcsh.el (source)...done
For information about GNU Emacs and the GNU system, type C-h C-a.
delete-backward-char: Text is read-only [3 times]

Configured using:
 'configure --build x86_64-linux-gnu --prefix=/usr
 --sharedstatedir=/var/lib --libexecdir=/usr/lib
 --localstatedir=/var/lib --infodir=/usr/share/info
 --mandir=/usr/share/man --enable-libsystemd --with-pop=yes
 --enable-locallisppath=/etc/emacs:/usr/local/share/emacs/27.1/site-lisp:/usr/local/share/emacs/site-lisp:/usr/share/emacs/27.1/site-lisp:/usr/share/emacs/site-lisp
 --with-sound=alsa --without-gconf --with-mailutils --build
 x86_64-linux-gnu --prefix=/usr --sharedstatedir=/var/lib
 --libexecdir=/usr/lib --localstatedir=/var/lib
 --infodir=/usr/share/info --mandir=/usr/share/man --enable-libsystemd
 --with-pop=yes
 --enable-locallisppath=/etc/emacs:/usr/local/share/emacs/27.1/site-lisp:/usr/local/share/emacs/site-lisp:/usr/share/emacs/27.1/site-lisp:/usr/share/emacs/site-lisp
 --with-sound=alsa --without-gconf --with-mailutils --with-x=no
 --without-gsettings 'CFLAGS=-g -O2
 -ffile-prefix-map=/build/emacs-LlFm6W/emacs-27.1+1=.
-fstack-protector-strong
 -Wformat -Werror=format-security -Wall' 'CPPFLAGS=-Wdate-time
 -D_FORTIFY_SOURCE=2' LDFLAGS=-Wl,-z,relro'

Configured features:
SOUND GPM DBUS NOTIFY INOTIFY ACL LIBSELINUX GNUTLS LIBXML2 ZLIB XIM
MODULES THREADS LIBSYSTEMD JSON PDUMPER LCMS2 GMP

Important settings:
  value of $LANG: C.UTF-8
  locale-coding-system: utf-8-unix

Major mode: Fundamental

Minor modes in effect:
  tooltip-mode: t
  global-eldoc-mode: t
  electric-indent-mode: t
  menu-bar-mode: t
  file-name-shadow-mode: t
  global-font-lock-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t
  buffer-read-only: t
  line-number-mode: t
  transient-mark-mode: t

Load-path shadows:
None found.

Features:
(shadow sort mail-extr emacsbug message rmc puny dired dired-loaddefs
format-spec rfc822 mml easymenu mml-sec password-cache epa derived epg
epg-config gnus-util rmail tool-bar rmail-loaddefs text-property-search
time-date subr-x seq mm-decode mm-bodies mm-encode mail-parse rfc2231
mailabbrev gmm-utils mailheader cl-loaddefs cl-lib sendmail rfc2047
rfc2045 ietf-drums mm-util mail-prsvr mail-utils term/xterm xterm
byte-opt gv bytecomp byte-compile cconv regexp-opt sudo-save tooltip
eldoc electric uniquify ediff-hook vc-hooks lisp-float-type
tabulated-list replace newcomment text-mode elisp-mode lisp-mode
prog-mode register page tab-bar menu-bar rfn-eshadow isearch timer
select mouse jit-lock font-lock syntax facemenu font-core
term/tty-colors frame minibuffer cl-generic cham georgian utf-8-lang
misc-lang vietnamese tibetan thai tai-viet lao korean japanese eucjp-ms
cp51932 hebrew greek romanian slovak czech european ethiopic indian
cyrillic chinese composite charscript charprop case-table epa-hook
jka-cmpr-hook help simple abbrev obarray cl-preloaded nadvice loaddefs
button faces cus-face macroexp files text-properties overlay sha1 md5
base64 format env code-pages mule custom widget hashtable-print-readable
backquote threads dbusbind inotify lcms2 multi-tty make-network-process
emacs)

Memory information:
((conses 16 48684 10446)
 (symbols 48 6056 1)
 (strings 32 15989 1774)
 (string-bytes 1 511028)
 (vectors 16 8200)
 (vector-slots 8 87466 8132)
 (floats 8 22 473)
 (intervals 56 216 0)
 (buffers 1000 12))

-- 
Regards
Cos Chan

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bug#50721: Wrong point position or display while moving point

[Eli Zaretskii]

> From: Cos Chan <rosettas@gmail.com>
> Date: Tue, 21 Sep 2021 14:26:44 +0200
> 
> To change system tab width by
> $ tabs 4
> would create a problem that the display is wrong while moving point (for example M-b/M-f/C-up/C-down or
> kill words/line).
> The point or the characters would be in the wrong places until I manually run M-x redraw-display to get it
> back.
> 
> I may reproduce this problem in Debian 10, 11 and FreeBSD 12, 13. Tested in zsh and tcsh by add tabs 4
> into .zshrc and .tcshrc

Thanks.

We initialize the tab width used for display on TTY frames from the
"tw" termcap/terminfo capability.  Does anyone know whether this is
updated by the 'tabs' command?  If not, is there a way Emacs could
know that the tab stops have been changed from their usual default?

If everything else fails, I guess we could provide a variable to
disable using TAB characters in cursor motion on TTYs, then users who
do the above could control how Emacs behaves in this case.