bug#55256: 27.1; Unicode noncharacters may change writing direction

[frederik . fouvry]

M-x set-input-method RET
ucs RET
jufdd0$1 RET

If you type C-a and then step through the characters with the right
arrow, the direction is reversed. It seems like the entry of $ followed
by something else is triggering it, but that may just be an
impression/side effect.

I suspect that the reason is that most Unicode noncharacters
(https://www.unicode.org/faq/private_use.html#nonchar1) are in an Arabic
block (https://www.unicode.org/faq/private_use.html#nonchar4b) and that
the cause is an incorrect generalisation of the properties of the
characters in this block.

In GNU Emacs 27.1 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.24.20, cairo 
version 1.16.0)
 of 2020-09-19 built on lgw01-amd64-021
Windowing system distributor 'The X.Org Foundation', version 11.0.12013000
System Description: Ubuntu 20.04.4 LTS

Recent messages:
Char: �‎ (64976, #o176720, #xfdd0, file ...) point=4361 of 7190 (61%) column=23
Mark set [2 times]
Auto-saving...done

Configured using:
 'configure --build=x86_64-linux-gnu --prefix=/usr
 '--includedir=${prefix}/include' '--mandir=${prefix}/share/man'
 '--infodir=${prefix}/share/info' --sysconfdir=/etc --localstatedir=/var
 --disable-silent-rules '--libdir=${prefix}/lib/x86_64-linux-gnu'
 '--libexecdir=${prefix}/lib/x86_64-linux-gnu' --disable-maintainer-mode
 --disable-dependency-tracking --prefix=/usr --sharedstatedir=/var/lib
 --libexecdir=/usr/lib --localstatedir=/var/lib
 --infodir=/usr/share/info --mandir=/usr/share/man
 
--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
 --program-suffix=27 --with-modules --with-file-notification=inotify
 --with-mailutils --with-harfbuzz --with-json --with-x=yes
 --with-x-toolkit=gtk3 --with-lcms2 --with-cairo --with-xpm=yes
 --with-gif=yes --with-gnutls=yes --with-jpeg=yes --with-png=yes
 --with-tiff=yes --with-xwidgets 'CFLAGS=-g -O2
 -fdebug-prefix-map=/build/emacs27-bifpWT/emacs27-27.1~1.git86d8d76aa3=. 
-fstack-protector-strong
 -Wformat -Werror=format-security -no-pie' 'CPPFLAGS=-Wdate-time
 -D_FORTIFY_SOURCE=2' 'LDFLAGS=-Wl,-Bsymbolic-functions -Wl,-z,relro
 -no-pie''

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

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

Major mode: Fundamental

Minor modes in effect:
  hexl-follow-ascii: t
  global-git-commit-mode: t
  magit-auto-revert-mode: t
  shell-dirtrack-mode: t
  global-activity-watch-mode: t
  activity-watch-mode: t
  auto-revert-mode: t
  show-paren-mode: t
  desktop-save-mode: t
  display-time-mode: t
  editorconfig-mode: t
  tooltip-mode: t
  global-eldoc-mode: t
  electric-indent-mode: t
  mouse-wheel-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
  transient-mark-mode: t

Load-path shadows:

Features:
(shadow sort mail-extr emacsbug ruler-mode hexl uni-input quail misearch
multi-isearch magit-submodule magit-obsolete magit-blame magit-stash
magit-reflog magit-bisect magit-push magit-pull magit-fetch magit-clone
magit-remote magit-commit magit-sequence magit-notes magit-worktree
magit-tag magit-merge magit-branch magit-reset magit-files magit-refs
magit-status magit magit-repos magit-apply magit-wip magit-log
which-func magit-diff smerge-mode diff git-commit magit-core
magit-autorevert magit-margin magit-transient magit-process with-editor
shell magit-mode transient magit-git magit-base magit-section crm dash
compat-27 compat-26 compat flymake-shellcheck flymake-proc flymake
compile sh-script executable css-mode smie imenu rng-xsd xsd-regexp
rng-cmpct rng-nxml rng-valid rng-loc rng-uri rng-parse nxml-parse
rng-match rng-dt rng-util rng-pttrn nxml-ns nxml-mode nxml-outln
nxml-rap sgml-mode nxml-util nxml-enc xmltok ol-eww eww mm-url url-queue
ol-rmail ol-mhe ol-irc ol-info ol-gnus nnir ol-docview ol-bibtex ol-bbdb
ol-w3m ol-doi org-link-doi vc-git diff-mode markdown-mode edit-indirect
color dired-aux server activity-watch-mode request autorevert filenotify
ert pp ewoc debug backtrace paren desktop frameset cus-start cus-load
auto-dictionary flyspell ispell time editorconfig-core
editorconfig-core-handle editorconfig-fnmatch timeclock mu4e mu4e-org
mu4e-main mu4e-view mu4e-view-gnus gnus-art mm-uu mml2015 mm-view
mml-smime smime dig gnus-sum url url-proxy url-privacy url-expand
url-methods url-history gnus-group gnus-undo gnus-start gnus-cloud
nnimap nnmail mail-source utf7 netrc nnoo parse-time iso8601 gnus-spec
gnus-int gnus-range gnus-win gnus nnheader wid-edit mu4e-view-common
thingatpt mu4e-headers mu4e-compose mu4e-context mu4e-draft mu4e-actions
ido rfc2368 smtpmail sendmail mu4e-mark mu4e-proc mu4e-utils doc-view
jka-compr image-mode exif mu4e-lists mu4e-message shr url-cookie
url-domsuf url-util svg xml dom flow-fill mule-util mailcap hl-line
mu4e-vars mu4e-meta dired-x calfw-org org-capture org-element avl-tree
generator org-agenda org-refile calfw holidays hol-loaddefs cl
org-journal edmacro kmacro org-crypt org ob ob-tangle ob-ref ob-lob
ob-table ob-exp org-macro org-footnote org-src ob-comint org-pcomplete
pcomplete comint ansi-color org-list org-faces org-entities noutline
outline org-version ob-emacs-lisp ob-core ob-eval org-table oc-basic
bibtex ol rx org-keys oc org-compat advice org-macs org-loaddefs
find-func cal-iso cal-menu calendar cal-loaddefs vc-svn dsvn log-edit
easy-mmode message rmc puny dired dired-loaddefs format-spec rfc822 mml
mml-sec epa derived epg epg-config gnus-util rmail rmail-loaddefs
warnings text-property-search time-date mm-decode mm-bodies mm-encode
mail-parse rfc2231 rfc2047 rfc2045 mm-util ietf-drums mail-prsvr
mailabbrev mail-utils gmm-utils mailheader ring pcvs-util add-log vc
vc-dispatcher editorconfig cl-extra help-mode use-package-ensure
use-package-core helm-easymenu info package easymenu browse-url
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 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 xwidget-internal cairo
move-toolbar gtk x-toolkit x multi-tty make-network-process emacs)

Memory information:
((conses 16 398834 33888)
 (symbols 48 37214 1)
 (strings 32 137584 6008)
 (string-bytes 1 4673915)
 (vectors 16 73119)
 (vector-slots 8 1675119 195896)
 (floats 8 531 329)
 (intervals 56 7085 0)
 (buffers 1000 76))