bug#37840: Missing in the Emacs manuals:

[Konrad Podczeck <konrad.podczeck@univie.ac.at>]

C-c C-c in this case runs the function "message-send-and-exit” from message.el:

(defun message-send-and-exit (&optional arg)
  "Send message like `message-send', then, if no errors, exit from mail buffer.
The usage of ARG is defined by the instance that called Message.
It should typically alter the sending method in some way or other."
  (interactive "P")
  (let ((buf (current-buffer))
	(actions message-exit-actions))
    (when (and (message-send arg)
	       (buffer-name buf))
      (message-bury buf)
      (if message-kill-buffer-on-exit
	  (kill-buffer buf))
      (message-do-actions actions)
      t)))

By “second instance” I meant the following situation:

Suppose originally I had frame A with buffer.foo and frame B with buffer.mail,

then after C-c  C-c	I get

frame A with buffer.foo and frame with B buffer.foo


> Am 28.10.2019 um 19:13 schrieb martin rudalics <rudalics@gmx.at>:
> 
> > Now if I have some buffer.foo open in its own frame, and then issue
> > C-x m, a new message buffer pops up in its own frame, as it should
> > be. However, after finally issuing C-c C-c to send the mail, the
> > frame which contained the mail buffer does not disappear (as it was
> > the case with the old special-display.regexp stuff), rather this
> > frame shows a second instance of buffer.foo.
> 
> Can you tell me the function(s) called by C-c C-c to get rid of the
> window or the mail buffer?  From what you say here, the function can
> be hardly 'kill-buffer' because that would not show "a second instance
> of buffer.foo".  And what is a second instance of a buffer?
> 
> martin