The chat lines disappear after a time, based on when they came into your chat window, so when nobody chats for a while the oldest line goes, then the next, etc., until finally there is no chat to show. DAoC has an option for this, as does AO. I gather FFXI is similar.
So basically, if a chat line is still visible after say... 30 seconds, it disappears. Also, it sounds like FFXI might have dynamic chat window sizing so it shrinks and expands vertically as the chat comes on and fades off.
I think that's what Trojake meant
