bug#33160: 26.1; force-mode-line-update interferes button highlight

[Fu Yuan <casouri@gmail.com>]

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Symptom: Highlight on buttons disappears when `force-mode-line-update` runs

Reproduce:
- emacs -q
- (run-at-time 0 0.3 'force-mode-line-update)
- M-x customize (for it has a bunch of buttons)
- Move cursor to a button, the highlight disappears in a blink

Sincerely, Yuan

In GNU Emacs 26.1 (build 1, x86_64-apple-darwin17.7.0, NS appkit-1561.60
Version 10.13.6 (Build 17G65))
 of 2018-08-21 built on missSilver
Windowing system distributor 'Apple', version 10.3.1671
Recent messages:
For information about GNU Emacs and the GNU system, type C-h C-a.

Configured using:
 'configure --disable-dependency-tracking --disable-silent-rules
 --enable-locallisppath=/usr/local/share/emacs/site-lisp
 --infodir=/usr/local/Cellar/emacs-plus/26.1/share/info/emacs
 --prefix=/usr/local/Cellar/emacs-plus/26.1 --with-xml2 --without-dbus
 --with-gnutls --with-imagemagick --with-modules --with-rsvg --with-ns
 --disable-ns-self-contained'

Configured features:
JPEG RSVG IMAGEMAGICK NOTIFY ACL GNUTLS LIBXML2 ZLIB TOOLKIT_SCROLL_BARS
NS MODULES THREADS LCMS2

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

Major mode: Fundamental

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
  blink-cursor-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t
  buffer-read-only: t
  line-number-mode: t
  transient-mark-mode: t

Load-path shadows:
None found.

Features:
(shadow sort mail-extr emacsbug message rmc puny seq byte-opt gv
bytecomp byte-compile cconv cl-loaddefs cl-lib dired dired-loaddefs
format-spec rfc822 mml easymenu mml-sec password-cache epa derived epg
epg-config gnus-util rmail rmail-loaddefs mm-decode mm-bodies mm-encode
mail-parse rfc2231 mailabbrev gmm-utils mailheader sendmail rfc2047
rfc2045 ietf-drums mm-util mail-prsvr mail-utils elec-pair time-date
tooltip eldoc electric uniquify ediff-hook vc-hooks lisp-float-type
mwheel term/ns-win ns-win ucs-normalize mule-util term/common-win
tool-bar dnd fontset image regexp-opt fringe tabulated-list replace
newcomment text-mode elisp-mode lisp-mode prog-mode register page
menu-bar rfn-eshadow isearch timer select scroll-bar mouse jit-lock
font-lock syntax facemenu font-core term/tty-colors frame 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 minibuffer
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 kqueue cocoa ns lcms2
multi-tty make-network-process emacs)

Memory information:
((conses 16 204151 9641)
 (symbols 48 20185 1)
 (miscs 40 70 162)
 (strings 32 28905 1351)
 (string-bytes 1 764039)
 (vectors 16 34995)
 (vector-slots 8 713652 9800)
 (floats 8 48 68)
 (intervals 56 220 0)
 (buffers 992 12))

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