bug#68801: 30.0.50; minibuffer-visible-completions=t makes RET in completion-in-region a no-op with nothing selected

[Spencer Baugh <sbaugh@janestreet.com>]

1. emacs -Q
2. In scratch:
(current- M-<tab>
3. Observe *Completions* pops up.
4. RET
5. *Completions* goes away and a newline is inserted.

Compare:

1. emacs -Q --eval '(setq minibuffer-visible-completions t)'
2. In scratch:
(current- M-<tab>
3. Observe *Completions* pops up.
4. RET
5. *Completions* goes away but no newline is inserted, and "No match" is
   printed.

I think the behavior with minibuffer-visible-completions=t is
unintuitive.  And it just duplicates what you can achieve with C-g
(closes *Completions*), so it's not very useful.

Even if I do:
(current- M-<tab> buf RET

which is sufficient to uniquely identify current-buffer, RET does not
complete to current-buffer, instead it just says "No match".

With minibuffer-visible-completions=t, I suggest RET with no completion
selected in completion-in-region should exit completion and insert a
newline.

Or maybe, slightly more radically, RET with no completion selected in
completion-in-region should select the first matching completion given
the current completion region?  At least one user expected that
behavior.

Another alternative is that RET doesn't close *Completions* at all,
which would match minibuffer behavior better.  I'm not a huge fan of
that, but it's more useful than the current behavior of RET.

Since there are a few alternatives, perhaps we could have a defcustom
for the RET behavior in completion-in-region.  I think the default
should be closing *Completions* and inserting a newline, since that
matches minibuffer-visible-completions=nil.

If this sounds reasonable I can write a patch implementing these.


In GNU Emacs 30.0.50 (build 24, x86_64-pc-linux-gnu, X toolkit, cairo
 version 1.15.12, Xaw scroll bars) of 2024-01-29 built on
 igm-qws-u22796a Repository revision:
 cbc8bdc1386a4bc9c420d8ab06b844c6f6928c35 Repository branch: master
 Windowing system distributor 'The X.Org Foundation', version
 11.0.12011000 System Description: Rocky Linux 8.9 (Green Obsidian)

Configured using:
 'configure -C 'CFLAGS=-O0 -g3' --with-gif=ifavailable
 --with-x-toolkit=lucid'

Configured features:
CAIRO DBUS FREETYPE GLIB GMP GNUTLS GSETTINGS HARFBUZZ JPEG JSON
LIBSELINUX LIBSYSTEMD LIBXML2 MODULES NOTIFY INOTIFY PDUMPER PNG RSVG
SECCOMP SOUND SQLITE3 THREADS TIFF TOOLKIT_SCROLL_BARS X11 XDBE XIM
XINPUT2 XPM LUCID ZLIB

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

Major mode: Lisp Interaction

Minor modes in effect:
  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
  line-number-mode: t
  indent-tabs-mode: t
  transient-mark-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t

Load-path shadows:
None found.

Features:
(shadow sort mail-extr dabbrev emacsbug message mailcap yank-media puny
dired dired-loaddefs rfc822 mml mml-sec password-cache epa derived epg
rfc6068 epg-config gnus-util text-property-search time-date subr-x
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 rmc iso-transl tooltip cconv eldoc paren
electric uniquify ediff-hook vc-hooks lisp-float-type elisp-mode mwheel
term/x-win x-win term/common-win x-dnd touch-screen 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 nadvice seq simple cl-generic
indonesian philippine 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 abbrev obarray oclosure cl-preloaded button loaddefs
theme-loaddefs faces cus-face macroexp files window text-properties
overlay sha1 md5 base64 format env code-pages mule custom widget keymap
hashtable-print-readable backquote threads dbusbind inotify
dynamic-setting system-font-setting font-render-setting cairo x-toolkit
xinput2 x multi-tty move-toolbar make-network-process emacs)

Memory information:
((conses 16 65716 19738) (symbols 48 9598 0) (strings 32 22938 2513)
 (string-bytes 1 684815) (vectors 16 9358)
 (vector-slots 8 112024 10425) (floats 8 42 15) (intervals 56 264 0)
 (buffers 976 10))