The issue, prior to this fix, is that the presence list actually
gets drawn *on top of* our "more messages" popup, which is ugly.
(Toggling the z-index on either or both of them did not seem
necessarily to matter, but that's probably because z-index
is subtler than I understand.)
This fixes Trac #1078.
(imported from commit a255aadb1884cf6c659085b26a36d378f680e83e)