bug#64903: 28.2; completion-at-point deletes next line(s) when completing a single character in elisp

[Thalis Charakas via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

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This bug happens with "emacs -q -Q"

When completing a single character with completion-at-point (M-C-i) in elsip,
after you select the desiered completion, it is inserted in the buffer
but some of the following lines are deleted. This does not happen when
the line has two characters.

Examples are with "emacs -q -Q"

Example 1:
- Write "(setq foo nil)" in scratch.
- Go to the line before that and type "(".
- Press M-C-i.
- Select any outecome.
The (setq foo nil) is no longer there.

Example 2:
- Open a .el file where the following code exists:
```
(use-package modus-themes
:config
;; Add all your customizations prior to loading the themes
(setq modus-themes-italic-constructs t
modus-themes-bold-constructs t
modus-themes-mixed-fonts t)

;; Maybe define some palette overrides, such as by using our presets
(setq modus-themes-common-palette-overrides
nil)
[...]
```
- Add a line after ":config" by navigating to the end of that line and
pressing RET.
- Type "(" in that new line and press M-C-i
- Select any outcome.
Everything up to the line before ";;Maybe define [...]" is deleted.

Example 3:

Doing the same as above but typing "(s" before pressing M-C-i.
After selecting an outcome, nothing is deleted.

Example 4:

- Open a .el file where the following code exists:
```
(use-package which-key
:init (setq which-key-idle-delay 0
which-key-idle-secondary-delay nil)
:config (which-key-mode 1))
```
- Add a line after the first (by pressing RET after "which-key"), and
type "(". Now the buffer is like this:
```
(use-package which-key
(
:init (setq which-key-idle-delay 0
which-key-idle-secondary-delay nil)
:config (which-key-mode 1))
```
- Press M-C-i and select anything, for example "abs".
- The file in now like this (":init" and a new-line are missing):
```
(use-package which-key
(abs (setq which-key-idle-delay 0
which-key-idle-secondary-delay nil)
:config (which-key-mode 1))
```

Some insight by the corfu maintainer:
https://github.com/minad/corfu/issues/350
(I first thought it was a corfu bug)

In GNU Emacs 28.2 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.24.37, cairo version 1.16.0)
of 2023-05-14, modified by Debian built on x86-ubc-01
Windowing system distributor 'The X.Org Foundation', version 11.0.12201009
System Description: Debian GNU/Linux 12 (bookworm)

Configured using:
'configure --build x86_64-linux-gnu --prefix=/usr
--sharedstatedir=/var/lib --libexecdir=/usr/libexec
--localstatedir=/var/lib --infodir=/usr/share/info
--mandir=/usr/share/man --with-libsystemd --with-pop=yes
--enable-locallisppath=/etc/emacs:/usr/local/share/emacs/28.2/site-lisp:/usr/local/share/emacs/site-lisp:/usr/share/emacs/28.2/site-lisp:/usr/share/emacs/site-lisp
--with-sound=alsa --without-gconf --with-mailutils
--with-native-compilation --build x86_64-linux-gnu --prefix=/usr
--sharedstatedir=/var/lib --libexecdir=/usr/libexec
--localstatedir=/var/lib --infodir=/usr/share/info
--mandir=/usr/share/man --with-libsystemd --with-pop=yes
--enable-locallisppath=/etc/emacs:/usr/local/share/emacs/28.2/site-lisp:/usr/local/share/emacs/site-lisp:/usr/share/emacs/28.2/site-lisp:/usr/share/emacs/site-lisp
--with-sound=alsa --without-gconf --with-mailutils
--with-native-compilation --with-cairo --with-x=yes
--with-x-toolkit=gtk3 --with-toolkit-scroll-bars 'CFLAGS=-g -O2
-ffile-prefix-map=/build/emacs-mPr7Vr/emacs-28.2+1=. -fstack-protector-strong
-Wformat -Werror=format-security -Wall' 'CPPFLAGS=-Wdate-time
-D_FORTIFY_SOURCE=2' LDFLAGS=-Wl,-z,relro'

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

Important settings:
value of $LC_MONETARY: el_GR.UTF-8
value of $LC_NUMERIC: el_GR.UTF-8
value of $LC_TIME: el_GR.UTF-8
value of $LANG: en_US.UTF-8
value of $XMODIFIERS: @im=ibus
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
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 completion comp comp-cstr warnings rx cl-extra
emacsbug message rmc puny dired dired-loaddefs rfc822 mml mml-sec epa
derived epg rfc6068 epg-config gnus-util rmail rmail-loaddefs
auth-source cl-seq eieio eieio-core cl-macs eieio-loaddefs
password-cache json map text-property-search seq byte-opt gv bytecomp
byte-compile cconv mm-decode mm-bodies mm-encode mail-parse rfc2231
mailabbrev gmm-utils mailheader sendmail rfc2047 rfc2045 ietf-drums
mm-util mail-prsvr mail-utils thingatpt help-fns radix-tree help-mode
time-date subr-x 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
native-compile emacs)

Memory information:
((conses 16 97960 20019)
(symbols 48 8156 0)
(strings 32 25004 1687)
(string-bytes 1 790041)
(vectors 16 17606)
(vector-slots 8 346516 15837)
(floats 8 32 45)
(intervals 56 346 0) (buffers 992 13))

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