bug#43033: 27.1; Two zero-width overlays with before- and after-string property

[Ikumi Keita]

I'm not sure this is really a bug or a feature, but here is the detail
anyway. When two zero-width overlays at the same position have before-
and after-string property respectively, emacs doesn't display those
strings in expected order.

[How to reproduce]
1. Save the following file as /tmp/foo.el:
--- /tmp/foo.el ----------------------------------
(setq ov1 (make-overlay (point-max) (point-max)))
(setq ov2 (make-overlay (point-max) (point-max)))
(overlay-put ov1 'after-string "AFTER")
(overlay-put ov2 'before-string "before")
--------------------------------------------------
2. emacs -Q -l /tmp/foo.el
3. Expected result: *scrach* buffer displays "beforeAFTER" in it
   Actual result: *scrach* buffer displays "AFTERbefore" in it

[How this behavior affects my usage of overlay]
When the minibuffer has a zero-width overlay at the EOB with
before-string property, `set-minibuffer-message' displays that string
after, not before, the echo area message shown by that function.

Regards,
Ikumi Keita


In GNU Emacs 27.1 (build 1, x86_64-unknown-freebsd12.1, GTK+ Version 3.24.20, 
cairo version 1.16.0)
 of 2020-08-20 built on freebsd.vmware
Windowing system distributor 'The X.Org Foundation', version 11.0.12008000
System Description: 12.1-RELEASE-p8

Recent messages:
Saving file /tmp/foo.el...
Wrote /tmp/foo.el
Making completion list...
Quit
Mark set [2 times]
Saving file /tmp/foo.el...
Wrote /tmp/foo.el
Undo [7 times]
Saving file /tmp/foo.el...
Wrote /tmp/foo.el

Configured using:
 'configure --with-canna --with-canna-includes=/usr/local/canna/include
--with-canna-libraries=/usr/local/canna/lib --without-xim
--disable-largefile --with-sound=yes --with-file-notification=yes
--with-cairo=yes --without-modules --with-xwidgets=yes CFLAGS=-O3'

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

Important settings:
  value of $EMACSLOADPATH: /home/keita/elisp:
  value of $LANG: ja_JP.eucJP
  locale-coding-system: japanese-iso-8bit-unix

Major mode: Org

Minor modes in effect:
  tooltip-mode: t
  global-eldoc-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
  line-number-mode: t
  auto-fill-function: org-auto-fill-function
  transient-mark-mode: t

Load-path shadows:
~/elisp/reftex-parse hides 
/usr/local/share/emacs/27.1/lisp/textmodes/reftex-parse

Features:
(shadow mh-identity mh-letter mh-comp emacsbug sendmail misearch
multi-isearch org-element avl-tree generator ol-eww ol-rmail ol-mhe
ol-irc ol-info ol-gnus nnir ol-docview doc-view image-mode exif
ol-bibtex bibtex ol-bbdb ol-w3m 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 ring org-list org-faces org-entities noutline outline
easy-mmode org-version ob-emacs-lisp ob-core ob-eval org-table ol
org-keys org-compat org-macs org-loaddefs find-func cal-menu calendar
cal-loaddefs mh-search shr-color color qp mh-alias multi-prompt crm
smiley mm-archive mail-extr mh-mime mh-gnus mh-show goto-addr thingatpt
gnus-cite gnus-art mm-uu mml2015 mm-view mml-smime smime dig gnus-sum
shr svg dom gnus-group gnus-undo gnus-start gnus-cloud nnimap nnmail
mail-source utf7 netrc nnoo gnus-spec gnus-int message format-spec
rfc822 mml mml-sec mm-decode mm-bodies mm-encode gmm-utils mailheader
gnus-win gnus-range gnus nnheader wid-edit mh-seq mh-inc hl-line
mh-tool-bar mh-xface mh-utils mh-folder which-func imenu gnus-util rmail
rmail-loaddefs mail-utils text-property-search mh-scan mh-e mh-compat
mailabbrev mh-buffers mh-loaddefs mule-util cl-extra help-mode
parse-time iso8601 time-date ucs-normalize epa derived epg epg-config
url-http url-auth mail-parse rfc2231 rfc2047 rfc2045 mm-util ietf-drums
mail-prsvr url-gw nsm rmc puny jka-compr warnings twittering-mode advice
url url-proxy url-privacy url-expand url-methods url-history url-cookie
url-domsuf url-util mailcap xml dired dired-loaddefs preview-latex
auto-loads tex-site canna-im 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 japan-util 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 kqueue 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 470707 33009)
 (symbols 48 24661 3)
 (strings 32 106353 4193)
 (string-bytes 1 3871565)
 (vectors 16 65764)
 (vector-slots 8 1209172 61320)
 (floats 8 448 226)
 (intervals 56 4593 597)
 (buffers 1000 28))