bug#38597: 26.3; M-x describe-key-briefly does not work with various keys in various modes

[Edward J. Sabol]

Steps to repoduce:
1. Open an RMAIL file.
2. M-x rmail-mode
3. Type `h' to generate rmail-summary window for the RMAIL file and
switch to the RMAIL-summary buffer/window.
4. Type `M-x describe-key-briefly' and then type `r'.

I would expect the "r" key to be described in the minibuffer, but it is not.
It only seems to work if you use `C-h c'. This is with `emacs -Q'.

This doesn't have anything to do with rmail-mode, I think. I'm just using it
to illustrate the issue.

This is a regression, but I'm not sure from when. I know `M-x
describe-key-briefly' works in Emacs 21.2.


In GNU Emacs 26.3 (build 2, x86_64-pc-linux-gnu, X toolkit)
 of 2019-11-19 built on heasarcdev
Windowing system distributor 'Scientific Linux', version 11.0.11704000
System Description:     Scientific Linux release 6.10 (Carbon)

Recent messages:
Checking 176 files in /software/usr/contrib/share/emacs/26.3/lisp/emacs-lisp...
Checking 24 files in /software/usr/contrib/share/emacs/26.3/lisp/cedet...
Checking 57 files in /software/usr/contrib/share/emacs/26.3/lisp/calendar...
Checking 87 files in /software/usr/contrib/share/emacs/26.3/lisp/calc...
Checking 105 files in /software/usr/contrib/share/emacs/26.3/lisp/obsolete...
Checking for load-path shadows...done
Type y, n, ! or SPC (the space bar): 
Please type y, n, ! or SPC (the space bar):  [3 times]
Quit
You can run the command ‘switch-to-buffer’ with C-x b
r runs the command rmail-summary-reply
Configured using:
 'configure --prefix=/software/usr/contrib
 --exec-prefix=/software/usr/contrib/linux-6.x --with-x-toolkit=lucid
 --with-xft=yes --with-toolkit-scroll-bars=no --with-gif=no
 --with-tiff=no --with-gnutls=no'

Configured features:
XAW3D XPM JPEG PNG SOUND GSETTINGS GLIB NOTIFY LIBSELINUX LIBXML2
FREETYPE XFT ZLIB LUCID X11 XDBE XIM THREADS

Important settings:
  value of $LANG: en_US.UTF-8
  value of $XMODIFIERS: @im=none
  locale-coding-system: utf-8-unix

Major mode: RMAIL Summary

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:
(novice pp shadow sort emacsbug sendmail mail-extr qp rmailsum help-mode
rmailmm 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 mm-decode
mm-bodies mm-encode mailabbrev gmm-utils mailheader mail-parse rfc2231
rmail rmail-loaddefs rfc2047 rfc2045 ietf-drums mm-util mail-prsvr
mail-utils elec-pair 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 inotify dynamic-setting font-render-setting x-toolkit
x multi-tty make-network-process emacs)

Memory information:
((conses 16 146727 30001)
 (symbols 48 20923 0)
 (miscs 40 6173 5232)
 (strings 32 37078 5051)
 (string-bytes 1 1368553)
 (vectors 16 15170)
 (vector-slots 8 530473 14410)
 (floats 8 53 251)
 (intervals 56 44328 4019)
 (buffers 992 19)
 (heap 1024 22266 1027))