bug#38199: 27.0.50; [Cairo] Display artifacts (small vertical colored bars)

[Bastian Beischer <bastian.beischer@gmail.com>]

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When emacs is built with cairo I see small display artifacts from time
to time, in particular there are small thin vertical bars, for example,
with the default theme (starting from emacs -Q) I get those bars in the
mode-line and they are always in a cyan color, but it is possible that
other occurences can be observed even with emacs -Q.

In real world usage I have seen these bars in regular buffers, too,
usually they are red and/or blue but that might depend on my theme.

A recipe to produce them (on my machine) is:

1) emacs -Q
2) M-<
3) C-space
4) M->
5) M-w
6) C-y (repeat ten or twenty times)

Then move around in the resulting buffer and after a bit of cursor
movement the vertical lines appear. They are not permanent and often
disappear when the buffer is refreshed.

This issue is not present when emacs is built without cairo.

I attached a screenshot of emacs showing a small vertical cyan bar in
the mode line. produced with the above recipe.


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In GNU Emacs 27.0.50 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.24.12, cairo version 1.17.3)
 of 2019-11-11 built on bastian-desktop
Repository revision: b6942c0c37a504e00c717c8c74bfa9dcd208c931
Repository branch: master
Windowing system distributor 'The X.Org Foundation', version 11.0.12005000
System Description: Arch Linux

Recent messages:
Redo [2 times]
Undo
Redo [2 times]
Undo [4 times]
funcall-interactively: End of buffer
Redo
Mark set
scroll-up-command: End of buffer
Mark set [27 times]
scroll-down-command: Beginning of buffer
Quit
Configured using:
 'configure --prefix=/usr --sysconfdir=/etc --libexecdir=/usr/lib
 --localstatedir=/var --mandir=/usr/share/man --with-gameuser=:games
 --with-sound=alsa --with-modules --without-gconf --without-gsettings
 --enable-link-time-optimization --with-x-toolkit=gtk3 --without-xaw3d
 --without-m17n-flt --with-cairo --without-compress-install
 'CFLAGS=-march=native -O2 -pipe -fno-plt -flto -fuse-linker-plugin
 -flto -fuse-linker-plugin -fuse-ld=gold' CPPFLAGS=-D_FORTIFY_SOURCE=2
 LDFLAGS=-Wl,-O1,--sort-common,--as-needed,-z,relro,-z,now'

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

Important settings:
  value of $LANG: en_US.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
  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 easymenu mml-sec password-cache epa derived epg
epg-config gnus-util rmail rmail-loaddefs text-property-search time-date
subr-x seq byte-opt gv bytecomp byte-compile cconv mm-decode mm-bodies
mm-encode mail-parse rfc2231 mailabbrev gmm-utils mailheader cl-loaddefs
cl-lib sendmail rfc2047 rfc2045 ietf-drums mm-util mail-prsvr mail-utils
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 dbusbind
inotify lcms2 dynamic-setting font-render-setting cairo move-toolbar gtk
x-toolkit x multi-tty make-network-process emacs)

Memory information:
((conses 16 48518 9761)
 (symbols 48 5992 1)
 (strings 32 16179 2235)
 (string-bytes 1 531927)
 (vectors 16 9976)
 (vector-slots 8 128959 10346)
 (floats 8 19 41)
 (intervals 56 538 0)
 (buffers 1000 12))