bug#40788: 28.0.50; Error running timer ‘custom-magic-reset’

[David Ponce <da_vid@orange.fr>]

Hello,

Here is the recipe for an error I encounter when using customize with
2 windows in the same frame:

emacs -Q
M-x pop-to-buffer RET *Message*
M-x customize-variable RET blink-cursor-alist

At this point the *scratch* buffer is on top window and the customize
buffer on bottom.

Click on 'INS' button and insert a new Key/Value pair
Clict on 'State' button, then 'Set for Current Session'

Then click somewhere in the *scratch* buffer, then click on the 'DEL'
button in the customize buffer to delete the value previously entered.

At this point custom try to redraw the 'State' button in the *scratch*
buffer instead of the customize buffer! In this case, because the
*scratch* buffer is shorter than the position of the State button, it
fails with the error:

Error running timer ‘custom-magic-reset’: (args-out-of-range #<buffer
*scratch*> #<marker (moves after insertion) at 287 in *Customize Option:
Blink Cursor Alist*> #<marker at 294 in *Customize Option: Blink Cursor
Alist*>)

This error can be severe because, if in place of the *scratch* buffer
you were editing a file big enough to include the position of the 'State'
button, the 'State' button is drawn in your file without you realizing it!
And if you ever realize it, this change cannot be undone!

Thanks!

In GNU Emacs 28.0.50 (build 32, x86_64-pc-linux-gnu, GTK+ Version 3.24.13, cairo version 1.16.0)
  of 2020-04-23 built
Repository revision: 7b15cc3ebb45e50ac5faf3bbc2a813afffdaa418
Repository branch: master
Windowing system distributor 'Fedora Project', version 11.0.12006000
System Description: Fedora 31 (KDE Plasma)

Recent messages:
Mark set
Creating customization items...
Creating customization items ...done
Creating customization setup...done
Mark set
Beginning of buffer
Mark set [2 times]
scroll-up-command: End of buffer

Configured using:
  'configure --prefix=/home/dponce --with-cairo
  PKG_CONFIG_PATH=/usr/local/lib/pkgconfig:/usr/lib/pkgconfig'

Configured features:
XPM JPEG TIFF GIF PNG RSVG CAIRO SOUND DBUS GSETTINGS GLIB NOTIFY
INOTIFY LIBSELINUX GNUTLS LIBXML2 FREETYPE HARFBUZZ M17N_FLT LIBOTF ZLIB
TOOLKIT_SCROLL_BARS GTK3 X11 XDBE XIM MODULES THREADS LIBSYSTEMD JSON
PDUMPER LCMS2 GMP

Important settings:
   value of $LC_TIME: fr_FR.utf8
   value of $LANG: fr_FR.UTF-8
   locale-coding-system: utf-8-unix

Major mode: Messages

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
   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 password-cache 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 vc-git
diff-mode easy-mmode bug-reference eieio-opt cl-extra seq speedbar
ezimage dframe cl-print debug backtrace find-func cus-theme eieio-custom
cl-seq eieio byte-opt bytecomp byte-compile cconv eieio-core cl-macs gv
eieio-loaddefs wid-browse tree-widget thingatpt help-fns radix-tree
help-mode cus-edit easymenu cus-start cus-load wid-edit time-date subr-x
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 dbusbind
inotify lcms2 dynamic-setting system-font-setting font-render-setting
cairo move-toolbar gtk x-toolkit x multi-tty make-network-process emacs)

Memory information:
((conses 16 98994 16077)
  (symbols 48 10003 1)
  (strings 32 26548 2003)
  (string-bytes 1 811951)
  (vectors 16 14342)
  (vector-slots 8 182195 11344)
  (floats 8 43 119)
  (intervals 56 4061 0)
  (buffers 992 16))