Shoutbox

I don't think it is about Messenger versions. It is related to Messenger Plus! version because this problem has been introduced after its 4.50 version because in 4.50 version, some other problems about tabbed chats has been fixed but unfortunately causing this problem.

So you can also upgrade to 8.5 and use warmth's [New Release] Old School Messenger 1.2 (with options) to use the old interface.