bug#22581: Strange minibuffer prompt on exiting Emacs

[Richard Copley <rcopley@gmail.com>]

The prompt to confirm exiting Ediff has gone a bit strange. To
reproduce in "emacs -Q" (assuming files "x" and "y" exist in the
working directory):

M-x ediff RET x RET y RET q

The expected prompt is
"Quit this Ediff session? (y or n)  ".

The actual text shown in the minibuffer is
"Quit this Ediff session? (y or n)  switch-frame-".

(The frame switch event occurred because the little Ediff control
panel frame went away and the main frame got activated.)

The "switch-frame-" is of course not really part of the prompt. It is
rendered like what you get when you enter an incomplete key sequence,
like "C-x-". It seems as though switch-frame is being treated as a
prefix key, but it's not treated as such in most circumstances. Try
this for example:

C-x 5 2 ;; make-frame-command
C-h k ;; describe-key
;; Use the window manager to activate the other frame.
;; Minibuffer prompt is "Describe key (or click or menu item):  switch-frame-")
RET
;; *Help* buffer describes "RET", not "switch-frame RET".

In GNU Emacs 25.0.90.1 (x86_64-w64-mingw32)
 of 2016-02-06 built on MACHINE
Repository revision: 87ae21858adc173c129ff8dfec8ce74cd82e77be
Windowing system distributor 'Microsoft Corp.', version 10.0.10586
Configured using:
 'configure --prefix /c/emacs/emacs-20160206-090820
 --without-imagemagick --disable-dependency-tracking
 --enable-locallisppath=%emacs_dir%/../site-lisp 'CFLAGS=-Og -g -ggdb''

Configured features:
XPM JPEG TIFF GIF PNG RSVG SOUND DBUS NOTIFY ACL GNUTLS LIBXML2 ZLIB
TOOLKIT_SCROLL_BARS

Important settings:
  value of $LANG: ENG
  locale-coding-system: cp1252

Major mode: Text

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
  line-number-mode: t
  transient-mark-mode: t

Recent messages:
For information about GNU Emacs and the GNU system, type C-h C-a.
Reading file c:/temp/xxx.txt ...
Reading file c:/temp/yyy.txt ...
Computing differences between xxx.txt and yyy.txt ...
Processing difference regions ... done
Quit this Ediff session? (y or n) y

Load-path shadows:
None found.

Features:
(shadow sort mail-extr emacsbug message dired format-spec rfc822 mml
mml-sec epg epg-config gnus-util mm-decode mm-bodies mm-encode
mail-parse rfc2231 mailabbrev gmm-utils mailheader sendmail rfc2047
rfc2045 ietf-drums mm-util help-fns help-mode easymenu mail-prsvr
mail-utils jka-compr ediff-merg ediff-wind ediff-diff ediff-mult
ediff-help ediff-init cl-loaddefs pcase cl-lib ediff-util ediff
time-date mule-util tooltip eldoc electric uniquify ediff-hook vc-hooks
lisp-float-type mwheel dos-w32 ls-lisp disp-table w32-win w32-vars
term/common-win tool-bar dnd fontset image regexp-opt fringe
tabulated-list newcomment elisp-mode lisp-mode prog-mode register page
menu-bar rfn-eshadow timer select scroll-bar mouse jit-lock font-lock
syntax facemenu font-core 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 charscript case-table epa-hook jka-cmpr-hook help
simple abbrev 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
w32notify dbusbind w32 multi-tty make-network-process emacs)

Memory information:
((conses 16 98162 11732)
 (symbols 56 20948 0)
 (miscs 48 47 181)
 (strings 32 18962 3451)
 (string-bytes 1 551173)
 (vectors 16 13332)
 (vector-slots 8 428898 4688)
 (floats 8 173 116)
 (intervals 56 244 12)
 (buffers 976 15))