As far as I can tell, there's no way to do what you're asking. When I "catch up" I use the Chatbox Archive and then go back to the Forum page to watch the scroll from there. You could use the Chatbox Full to catch up, but I wonder if it wouldn't still do the same thing that's bothering you. I don't use the Chatbox Full setting unless I put it in its own tab while I'm doing a lot of forum reading.