bug#46243: 26.3; If invoke menu item that reads a char, get keystrokes echo

[Drew Adams]

There's no doubt a simpler thing to repro this with.  Sorry, I'm pressed
for time.

I see this starting with Emacs 24, and still in Emacs 27.1.  There's no
such problem before Emacs 24.

If I have a menu item that reads a character, the prompt for that read
is not seen; the menu selection key sequence gets echoed instead.  This
shouldn't happen (should it?).

Binding `echo-keystrokes' in the command's `interactive' spec doesn't
prevent this.  The workaround I've found is to add a call to (message
nil).

Example command:

(defun diredp-mark-with-char (char &optional arg)
  "Mark this line with CHAR.
With numeric prefix arg N, mark the next N lines."
  (interactive
   (progn (message nil)
          (list (read-char "Mark this line with char: ")
                (prefix-numeric-value current-prefix-arg))))
  (let ((dired-marker-char  char))
    (dired-mark arg)))

If the `interactive' spec does not have that (message nil) then it
doesn't work - you then see the menu-invocation key sequence instead of
the prompt.  And this is so whether I use "cChar: \np" or a sexp with
`read-char'.

If I change to reading a string then there's no such problem:

(defun diredp-mark-with-char (char &optional arg)
  "Mark this line with CHAR.
With numeric prefix arg N, mark the next N lines."
  (interactive "sChar: \np")
  (let ((dired-marker-char  (string-to-char char)))
    (dired-mark arg)))

So that's another possible workaround.  (Is one of those better?)

Haven't been able to see why this problem occurs.

This is the menu item I have for the above command:

(define-key diredp-marks-mark-menu [diredp-mark-with-char]
  '(menu-item "Mark This with Char..." diredp-mark-with-char
              :visible (not (diredp-nonempty-region-p))
              :help "Mark this line with a character you type"))

I see the problem also with `easy-menu-create-menu':

(easy-menu-create-menu
 "This File"
 '(["Mark with Char..." diredp-mark-with-char]))

Please advise.  Am I missing something?  As I say, this was not a
problem before Emacs 24.

In GNU Emacs 26.3 (build 1, x86_64-w64-mingw32)
 of 2019-08-29
Repository revision: 96dd0196c28bc36779584e47fffcca433c9309cd
Windowing system distributor `Microsoft Corp.', version 10.0.18362
Configured using:
 `configure --without-dbus --host=x86_64-w64-mingw32
 --without-compress-install 'CFLAGS=-O2 -static -g3''