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))
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