bug#51550: 29.0.50; Customize Group INS buttons sometimes don't have a left box line

[Po Lu]

Run emacs -Q.  Do M-x customize-group RET files RET, expand "After Save
Hook", then click "INS" twice.  The second INS button will not have a
left box line, which is visually inconsistent with the first, and with
how buttons work in general across all X-Windows applications.

The repository branch used is not relevant here, it can also be
reproduced in master and emacs-28.

Thanks.

In GNU Emacs 29.0.50 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.24.30, cairo 
version 1.17.4)
 of 2021-11-01 built on limity
Repository branch: x-window-xwidget
Windowing system distributor 'The X.Org Foundation', version 11.0.12101002
System Description: Fedora 34 (Workstation Edition)

Configured using:
 'configure --with-xwidgets'

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

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

Major mode: Custom

Minor modes in effect:
  shell-dirtrack-mode: t
  tooltip-mode: t
  global-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 emacsbug message rmc puny rfc822 mml mml-sec epa
derived gnus-util rmail rmail-loaddefs text-property-search mm-decode
mm-bodies mm-encode mail-parse rfc2231 mailabbrev gmm-utils mailheader
sendmail rfc2047 rfc2045 ietf-drums mm-util mail-prsvr mail-utils
shadowfile tramp tramp-loaddefs trampver tramp-integration files-x
tramp-compat shell pcomplete parse-time iso8601 time-date ls-lisp
format-spec auth-source cl-seq eieio eieio-core cl-macs eieio-loaddefs
password-cache json map recentf tree-widget plstore epg rfc6068
epg-config latexenc filecache dired dired-loaddefs autorevert filenotify
autoinsert seq gv subr-x byte-opt bytecomp byte-compile cconv ange-ftp
comint ansi-color ring cus-edit pp cus-start cus-load wid-edit help-mode
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 xwidget-internal dbusbind
inotify lcms2 dynamic-setting system-font-setting font-render-setting
cairo move-toolbar gtk x-toolkit x multi-tty make-network-process emacs)

Memory information:
((conses 16 100745 13815)
 (symbols 48 10589 1)
 (strings 32 32135 2089)
 (string-bytes 1 1038099)
 (vectors 16 19747)
 (vector-slots 8 238756 13660)
 (floats 8 41 129)
 (intervals 56 635 172)
 (buffers 992 13))