bug#50660: 28.0.50; Text artifacting when the cursor moves over text under mouse face that originally displayed a box

[Po Lu]

Start Emacs with -Q, run `list-faces-display', move to a face with a box
defined (for example, mode-line-highlight), highlight the sample text
with the mouse, and move the cursor around inside the sample text.

If you're lucky, you will be able to observe artifacting, usually around
the characters "O", "Q", "R", "W", "X", "Y", and/or "Z".  If you're not,
you can usually at least observe the text being redrawn in a manner
inconsistent with its surroundings, where characters surrounding the
cursor appear to be spaced overly close to or overly far from their
surroundings.

I've attached a screenshot depicting the results of moving the cursor to
the end of the sample text for mode-line-highlight while under mouse
face, and then quickly moving the cursor to the next line.  Note the
artifacting to the left of the character "Z", and how "Z" appears to be
spaced further apart than its neighbors to the left.

artifacting.png
Description: PNG image

In GNU Emacs 28.0.50 (build 1, i386-pc-solaris2.11, X toolkit, Xaw scroll bars)
 of 2021-09-18 built on blaster
Repository revision: 93731cdae0ff11d9f67e06ed3ea75b1320ce5034
Repository branch: HEAD
Windowing system distributor 'The X.Org Foundation', version 11.0.12101001
Configured using:
 'configure --with-gnutls=ifavailable --disable-acl'

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

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

Major mode: Help

Minor modes in effect:
  tooltip-mode: t
  global-eldoc-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
  buffer-read-only: 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 dired dired-loaddefs
rfc822 mml mml-sec epa derived epg rfc6068 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 misearch
multi-isearch help-mode info sly-autoloads package browse-url url
url-proxy url-privacy url-expand url-methods url-history url-cookie
url-domsuf url-util mailcap 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
iso-transl 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 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 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
dynamic-setting x-toolkit x multi-tty make-network-process emacs)

Memory information:
((conses 8 70437 15727)
 (symbols 24 7754 1)
 (strings 16 24397 1118)
 (string-bytes 1 758560)
 (vectors 8 16650)
 (vector-slots 4 258394 24378)
 (floats 8 38 22)
 (intervals 28 786 0)
 (buffers 564 13))