From http://firstname.lastname@example.org :
"Sometime the text I receive is fairly long and will appear on many
visual lines. It is generally nicely wrapped so I'm fairly happy with
that, but in this case the typing area goes *below* the bottom of the
window. I mean the "---" line is the *last* visible line. Because emacs
can not handle the situation where the point is out of the visible area,
it automaticaly put it on the line half-height, in the read-only area.
So I have to M-> , then type some text and send it."
I can more or less reproduce it, but only inside the chat buffer. Needs more investigation...