bug#50402: 27.0.90; column-number-mode breaks Farsi/Arabic character shaping

[Mohammad Razavi]

As you may know in scripts such as Farsi (Persian) or Arabic characters
may change shape depending on their adjacent characters
(http://www.unicode.org/versions/Unicode1.0.0/V2appA.pdf).


This behavior works fine in emacs; but if you enable
"column-number-mode" and copy/paste Farsi/Arabic script into the buffer
character shaping will not work correctly.


To reproduce the problem, you can run


emacs -q --eval '(progn (setq column-number-mode t) (switch-to-buffer
"foobar"))'


and then copy (only one of) the strings from below:


افغانستان


ایران


(The first word is "Afghanistan" and the second one is "Iran" in Farsi
script)


and paste it on the emacs buffer. You will see:


ﺎﻔﻏﺎﻨﺴﺗﺎﻧ


ﺎﯾﺭﺎﻧ


in which the character shaping is broken. If it didn't work with this
simple words try long Farsi/Arabic texts.


This problem only exists if  column-number-mode is enabled. Strangely,
if you type the words usually it works fine but if you copy/paste from
somewhere else it will not work. Also if you type some word and then
copy/paste the same work the character shaping works fine.



In GNU Emacs 27.0.90 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.24.14)
of 2020-03-29 built on 30bc0080ed46
Repository revision: c5f255d68156926923232b1edadf50faac527861
Repository branch: HEAD
Windowing system distributor 'The X.Org Foundation', version 11.0.12011000
System Description: Ubuntu 20.04.3 LTS

Recent messages:
For information about GNU Emacs and the GNU system, type C-h C-a.
Making completion list...

Configured using:
'configure --build x86_64-linux-gnu --prefix=/opt/emacs
--with-mailutils=yes --with-sound=alsa --without-gconf --with-x=yes
--with-x-toolkit=gtk3 --with-toolkit-scroll-bars 'CFLAGS=-g -O2
-fstack-protector-strong -Wformat -Werror=format-security -Wall'
'CPPFLAGS=-Wdate-time -D_FORTIFY_SOURCE=2'
'LDFLAGS=-Wl,-Bsymbolic-functions -Wl,-z,relro''

Configured features:
XPM JPEG TIFF GIF PNG RSVG SOUND GPM DBUS GSETTINGS GLIB NOTIFY INOTIFY
ACL LIBSELINUX GNUTLS LIBXML2 FREETYPE HARFBUZZ M17N_FLT LIBOTF XFT ZLIB
TOOLKIT_SCROLL_BARS GTK3 X11 XDBE XIM MODULES THREADS LIBSYSTEMD PDUMPER
LCMS2 GMP

Important settings:
value of $LANG: en_US.UTF-8
value of $XMODIFIERS: @im=ibus
locale-coding-system: utf-8-unix

Major mode: Fundamental

Minor modes in effect:
tooltip-mode: t
global-eldoc-mode: t
electric-indent-mode: t
mouse-wheel-mode: t
tool-bar-mode: t
menu-bar-mode: t
file-name-shadow-mode: t
global-font-lock-mode: t
font-lock-mode: t
blink-cursor-mode: t
auto-composition-mode: t
auto-encryption-mode: t
auto-compression-mode: t
column-number-mode: 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 rmail-loaddefs text-property-search time-date
subr-x seq byte-opt gv bytecomp byte-compile cconv 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
tooltip eldoc electric uniquify ediff-hook vc-hooks lisp-float-type
mwheel term/x-win x-win term/common-win x-dnd tool-bar dnd fontset image
regexp-opt fringe tabulated-list replace newcomment text-mode elisp-mode
lisp-mode prog-mode register page tab-bar menu-bar rfn-eshadow isearch
timer select scroll-bar 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 dynamic-setting system-font-setting font-render-setting
move-toolbar gtk x-toolkit x multi-tty make-network-process emacs)

Memory information:
((conses 16 45662 7148)
(symbols 48 5981 1)
(strings 32 16013 1905)
(string-bytes 1 518187)
(vectors 16 10056)
(vector-slots 8 129239 10066)
(floats 8 20 39)
(intervals 56 214 0)
(buffers 1000 13))