bug#42621: 26.3; Scheme mode: Wrong indentation (no indentation) when pressing tab

[Kjetil Matheussen <k.s.matheussen@gmail.com>]

Recipe to reproduce:
(note that it's important that you do _exactly_ as described. The bug
happens very often (and is extremely annoying), but it's very unclear
what triggers it.)

1. wget http://users.notam02.no/~kjetism/emacsindentationbug.scm
2. emacs emacsindentationbug.scm
3. Press Ctrl + S, search for "define column-widths"
4. The cursor should now be placed at line 2790
5. Press down-arrow so that that cursor is placed at line 2791
6. Press up -arrow so that the cursor is placed at line 2790 again.
7. Press TAB.
8. The line is wrongly indented (no indentation).


In GNU Emacs 26.3 (build 1, x86_64-pc-linux-gnu, X toolkit, Xaw scroll bars)
 of 2020-07-18 built on localhost.localdomain
Windowing system distributor 'Fedora Project', version 11.0.11405000
System Description: Fedora release 20 (Heisenbug)

Recent messages:
Checking 24 files in /home/kjetil/site/share/emacs/26.3/lisp/cedet...
Checking 57 files in /home/kjetil/site/share/emacs/26.3/lisp/calendar...
Checking 87 files in /home/kjetil/site/share/emacs/26.3/lisp/calc...
Checking 105 files in /home/kjetil/site/share/emacs/26.3/lisp/obsolete...
Checking for load-path shadows...done
previous-line: Beginning of buffer [3 times]
Auto-saving...done
previous-line: Beginning of buffer [5 times]
Buffer *unsent mail to bug-gnu-emacs@gnu.org* modified; kill anyway? (y or n) y
Mark saved where search started [2 times]

Configured using:
 'configure --prefix=/home/kjetil/site --with-x-toolkit=lucid'

Configured features:
XPM JPEG TIFF GIF PNG SOUND DBUS GSETTINGS GLIB NOTIFY LIBSELINUX GNUTLS
LIBXML2 FREETYPE XFT ZLIB TOOLKIT_SCROLL_BARS LUCID X11 XDBE XIM THREADS

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

Major mode: Scheme

Minor modes in effect:
  diff-auto-refine-mode: t
  iswitchb-mode: t
  shell-dirtrack-mode: t
  tooltip-mode: t
  global-eldoc-mode: t
  electric-indent-mode: t
  mouse-wheel-mode: t
  menu-bar-mode: t
  file-name-shadow-mode: t
  global-font-lock-mode: t
  font-lock-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t
  column-number-mode: t
  line-number-mode: t
  transient-mark-mode: t

Load-path shadows:
None found.

Features:
(pp shadow sort mail-extr emacsbug message rmc puny rfc822 mml mml-sec
epa derived epg gnus-util rmail rmail-loaddefs mm-decode mm-bodies
mm-encode mail-parse rfc2231 mailabbrev gmm-utils mailheader help-fns
radix-tree align two-column iso-transl help-mode xref project ffap
thingatpt make-mode dired-aux dired dired-loaddefs cmuscheme conf-mode
cc-mode cc-fonts cc-guess cc-menus cc-cmds misearch multi-isearch vc-git
diff-mode easy-mmode scheme elec-pair color-theme wid-edit cl sendmail
rfc2047 rfc2045 ietf-drums mm-util mail-prsvr mail-utils reporter
iswitchb finder-inf scala-mode2-autoloads package epg-config
url-handlers url-parse url-vars python easymenu tramp-sh tramp
tramp-compat tramp-loaddefs trampver ucs-normalize shell pcomplete
parse-time format-spec advice auth-source cl-seq eieio eieio-core
cl-macs eieio-loaddefs password-cache json map seq byte-opt gv bytecomp
byte-compile cconv comint ring ansi-color edmacro kmacro cc-styles
cc-align cc-engine cc-vars cc-defs cl-loaddefs cl-lib time-date
mule-util 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 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 threads dbusbind
inotify dynamic-setting system-font-setting font-render-setting
x-toolkit x multi-tty make-network-process emacs)

Memory information:
((conses 16 1270767 168270)
 (symbols 48 37217 2)
 (miscs 40 934 9001)
 (strings 32 113664 7701)
 (string-bytes 1 3016589)
 (vectors 16 60156)
 (vector-slots 8 1962000 165392)
 (floats 8 227 660)
 (intervals 56 160970 1318)
 (buffers 992 116)
 (heap 1024 108471 4922))