Subject: bug#79026: 30.1; TAB in Minibuffer when using a quail input-method should complete the longest common suffix of candidates

From: Cass Alexandru <[email-masked]>
Date: Tue, 15 Jul 2025 08:21:56 +0000

1. Steps to reproduce: Type the following in a buffer with `Fundamental` 
mode:
```
M-x set-input-method RET TeX RET
\ab
```
2. Expected behaviour: `TAB` completes to the only possible option 
(\above) and inserts `┴`.

3. Actual behaviour: `TAB` pops up the `*Quail Completions*` buffer with 
the following content, and I need to fully type out `\above` for `┴` to 
be inserted in the main buffer:
```
Possible completion and corresponding characters are:
\ab: -
\abo: -
\abov: -
\above:(1/1) 1.┴
```

My question is: How can I get the expected behaviour instead of the 
observed one?

In GNU Emacs 30.1 (build 1, x86_64-pc-linux-gnu, X toolkit, cairo
version 1.18.2, Xaw3d scroll bars)
Windowing system distributor 'The X.Org Foundation', version 11.0.12101016
System Description: NixOS 25.05 (Warbler)

Configured using:
'configure
--prefix=/nix/store/glh7dkancz0wlk5gh6zii601f0alys2v-emacs-30.1
--disable-build-details --with-modules --with-x-toolkit=lucid
--with-cairo --without-xft --with-compress-install
--with-toolkit-scroll-bars --with-native-compilation
--without-imagemagick --with-mailutils --without-small-ja-dic
--with-tree-sitter --with-xinput2 --without-xwidgets --with-dbus
--with-selinux'

Configured features:
CAIRO DBUS FREETYPE GIF GLIB GMP GNUTLS GPM GSETTINGS HARFBUZZ JPEG
LIBOTF LIBSELINUX LIBSYSTEMD LIBXML2 M17N_FLT MODULES NATIVE_COMP NOTIFY
INOTIFY PDUMPER PNG RSVG SECCOMP SOUND SQLITE3 THREADS TIFF
TOOLKIT_SCROLL_BARS TREE_SITTER WEBP X11 XAW3D XDBE XIM XINPUT2 XPM
LUCID ZLIB

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

Major mode: ELisp/l

Minor modes in effect:
global-git-commit-mode: t
magit-auto-revert-mode: t
show-smartparens-global-mode: t
show-smartparens-mode: t
smartparens-mode: t
server-mode: t
desktop-save-mode: t
TeX-PDF-mode: t
winner-mode: t
ido-everywhere: t
yas-global-mode: t
yas-minor-mode: t
override-global-mode: t
tooltip-mode: t
global-eldoc-mode: t
eldoc-mode: t
show-paren-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
minibuffer-regexp-mode: t
buffer-read-only: t
line-number-mode: t
transient-mark-mode: t
auto-composition-mode: t
auto-encryption-mode: t
auto-compression-mode: t

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Memory information:
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(strings 32 457206 25309) (string-bytes 1 17653278)
(vectors 16 144083) (vector-slots 8 2460867 369307)
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