The Posframe displays the minibuffer, but it doesn't have a minibuffer.
The actual minibuffer is still in the original frame, but it appears as
if it were not there because the mini-window's vscroll is set to
disguise that fact.
The trick doesn't work on ttys simply because I didn't implement vscroll
for ttys in Emacs 21.
Ahh apologies, so that first issue is a case of me misremembering what the previous tty behaviour was like, and it's really the multiple clients cases that are problematic (although in a different way to when this bug was first opened).
Cheers,
Len.