bug#61269: 28.2; Sequence of spaces preceding tab in bidirectional line

[Halim <mhalimln@outlook.com>]

  In a left-to-right line emacs display a sequence of one or more
spaces (U+0020), where the spaces precede a tab (U+0009) and they
both appear between two right-to-left alphabet, to the left of the
first (in typing order) rtl alphabet.

  The bug does not present when the rtl text is inside an rtl
isolate.

  Let s represent space, t represet tab, l represent itself, r and
m represent arabic alphabet. The following example have this format
in typing order from left to right.

Format:
lsrssstm

Example text:
l ح   	م

  The expected display is 'lsrssstm', the actual is 'lssssrtm'.
The spaces following 'r' in the format is displayed to the left
of 'r' in the actual display. Using 'C-f' from 'r' moves the
cursor to the left until it hits 't' where the cursor move to
the right of 'r'.

  I have tried to view the file containing the buggy text in
focuswriter and fribidi. They both display the same expected
way.

Extra Info

  The bug also present to ltr text on rtl line. I believe
this is generic and is caused by this line
'&& level != bidi_it->level_stack[0].level' (see below).

  The bug also present in emacs built from commit
'ac7ec87a7a0db887e4ae7fe9005aea517958b778' with
--without-all. In this commit I make the following
modification.

---------------
$ git diff ac7ec87a7a0db887e4ae7fe9005aea517958b778
diff --git a/src/bidi.c b/src/bidi.c
index e012512..fe6e4d6 100644
--- a/src/bidi.c
+++ b/src/bidi.c
@@ -3302,10 +3302,7 @@ bidi_level_of_next_char (struct bidi_it *bidi_it)
   if ((bidi_it->orig_type == NEUTRAL_WS
        || bidi_it->orig_type == WEAK_BN
        || bidi_isolate_fmt_char (bidi_it->orig_type))
-      && bidi_it->next_for_ws.charpos < bidi_it->charpos
-      /* If this character is already at base level, we don't need to
-        reset it, so avoid the potentially costly loop below.  */
-      && level != bidi_it->level_stack[0].level)
+      && bidi_it->next_for_ws.charpos < bidi_it->charpos)
     {
       int ch;
       ptrdiff_t clen = bidi_it->ch_len;
---------------

It fixes the bug.
  

In GNU Emacs 28.2 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.24.36, cairo version 1.17.6)
 of 2023-01-03 built on 2
Windowing system distributor 'The X.Org Foundation', version 11.0.12101006
System Description: Arch Linux

Configured using:
 'configure --sysconfdir=/etc --prefix=/usr --libexecdir=/usr/lib
 --localstatedir=/var --with-cairo --with-harfbuzz --with-libsystemd
 --with-modules --with-x-toolkit=gtk3 'CFLAGS=-march=x86-64
 -mtune=generic -O2 -pipe -fno-plt -fexceptions -Wp,-D_FORTIFY_SOURCE=2
 -Wformat -Werror=format-security -fstack-clash-protection
 -fcf-protection -g
 -ffile-prefix-map=/build/emacs/src=/usr/src/debug/emacs -flto=auto'
 'LDFLAGS=-Wl,-O1,--sort-common,--as-needed,-z,relro,-z,now -flto=auto''

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

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

Major mode: Fundamental

Minor modes in effect:
  delete-selection-mode: t
  cua-mode: t
  umath-mode: umath-insert-common
  tooltip-mode: t
  global-eldoc-mode: t
  show-paren-mode: t
  electric-indent-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
  line-number-mode: t
  indent-tabs-mode: t
  transient-mark-mode: t

Load-path shadows:
None found.

Features:
(shadow sort mail-extr emacsbug sendmail misearch multi-isearch
mule-util jka-compr nndraft nnmh nnfolder utf-7 rfc2104 gnutls
gnus-agent gnus-srvr gnus-score score-mode nnvirtual gnus-msg gnus-art
mm-uu mml2015 mm-view mml-smime smime dig nntp gnus-cache gnus-sum shr
kinsoku svg dom gnus-group gnus-undo gnus-start gnus-dbus dbus xml
gnus-cloud nnimap nnmail mail-source utf7 netrc nnoo parse-time iso8601
gnus-spec gnus-int gnus-range message dired dired-loaddefs rfc822 mml
mml-sec epa mm-decode mm-bodies mm-encode mail-parse rfc2231 mailabbrev
gmm-utils mailheader gnus-win gnus nnheader gnus-util rmail
rmail-loaddefs rfc2047 rfc2045 ietf-drums time-date mail-utils mm-util
mail-prsvr display-fill-column-indicator display-line-numbers delsel
cua-base cus-load lsp-mode lsp-protocol help-mode xref project
tree-widget wid-edit spinner pcase network-stream puny nsm rmc
markdown-mode rx color thingatpt noutline outline lv inline imenu ht
filenotify f f-shortdoc shortdoc s ewoc epg rfc6068 epg-config dash
compile text-property-search comint ansi-color ring finder-inf edmacro
kmacro easy-mmode derived info cl package browse-url url url-proxy
url-privacy url-expand url-methods url-history url-cookie url-domsuf
url-util mailcap url-handlers url-parse auth-source cl-seq eieio
eieio-core cl-macs eieio-loaddefs password-cache json subr-x map
url-vars seq byte-opt gv bytecomp byte-compile cconv cl-loaddefs cl-lib
iso-transl tooltip eldoc paren electric uniquify ediff-hook vc-hooks
lisp-float-type elisp-mode 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 lisp-mode prog-mode register page tab-bar menu-bar
rfn-eshadow isearch easymenu timer select scroll-bar mouse jit-lock
font-lock syntax 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 emoji-zwj charscript
charprop case-table epa-hook jka-cmpr-hook help simple abbrev obarray
cl-preloaded nadvice button loaddefs faces cus-face macroexp files
window 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
cairo move-toolbar gtk x-toolkit x multi-tty make-network-process emacs)

Memory information:
((conses 16 386790 21130)
 (symbols 48 30110 6)
 (strings 32 132616 6853)
 (string-bytes 1 3608021)
 (vectors 16 51861)
 (vector-slots 8 610382 31136)
 (floats 8 356 324)
 (intervals 56 4882 0)
 (buffers 992 21))