bug#37840: Missing in the Emacs manuals:

[martin rudalics]

> The entry (dedicated . t) did the job. (BTW, where is this possibility 
documented?)

In section 28.15 Dedicated Windows of the Elisp manual.  But as I said
in my first answer in this thread, indiscriminately applying a non-nil
'dedicated' entry is not generally advisable since it might affect
windows that pop up on the same frame too, with the consequence that
you can no longer switch buffers in such windows.  And it won't handle
the case where the buffer is shown in the selected window either.

As explained in section 28.16 Quitting Windows of the Elisp manual,
'quit-restore-window' could automatically DTRT for windows like that
used for composing mail, so there would be no need for a 'dedicated'
entry.  But 'message-send-and-exit' calls 'message-bury' and that uses
a slightly weird strategy to possibly get rid of the window with the
consequence that 'quit-restore-window' won't even get called here.

A zache Gschicht, so to say.

martin