bug#50424: 27.2; Tab bar button mouse face not clearing entirely

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

Enable tab-bar-mode, move the mouse pointer over the "+" button used to
create new tabs.  Once the button is highlighted by displaying a relief,
move the pointer back outside the tab bar.

While most the relief around part of the button will be cleared, part of
the left side of the relief that surrounded the "+" button when it was
highlighted will be retained.

To the best of my knowledge, this also appears on Emacs 28.

In GNU Emacs 27.2 (build 1, i386-pc-solaris2.11, X toolkit, Xaw scroll bars)
 of 2021-06-25 built on saphire
Windowing system distributor 'The X.Org Foundation', version 11.0.12101001
Recent messages:
For information about GNU Emacs and the GNU system, type C-h C-a.
Tab-Bar mode enabled

Configured using:
 'configure --with-gnutls=ifavailable'

Configured features:
XPM JPEG TIFF GIF PNG ACL ZLIB TOOLKIT_SCROLL_BARS LUCID X11 XDBE XIM
MODULES THREADS PDUMPER

Important settings:
  value of $LANG: en_GB.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
  electric-indent-mode: t
  mouse-wheel-mode: t
  tool-bar-mode: t
  tab-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
  transient-mark-mode: t

Load-path shadows:
None found.

Features:
(shadow sort mail-extr emacsbug message rmc puny dired dired-loaddefs
format-spec rfc822 mml mml-sec epa derived epg epg-config 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 time-date sly-autoloads
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
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 dynamic-setting
x-toolkit x multi-tty make-network-process emacs)

Memory information:
((conses 8 59679 8162)
 (symbols 24 7234 1)
 (strings 16 20406 2463)
 (string-bytes 1 658456)
 (vectors 8 12821)
 (vector-slots 4 211903 12844)
 (floats 8 26 20)
 (intervals 28 221 7)
 (buffers 568 13))