Not sure if there's a thread on this, but doesn't really matter anymore anyway.

It's been happening weeks for me now, first I thought it was just Tig's standalone that did it, because it didn't happen to me before I couldn't use Baff's new beta standalone. An original factor to most of the chat cutting off was if I clicked on the + button, which enlarges the chat box. But now it happens randomly, and even on Baff's old standalone...

Edit: Happens on the main site too.

I think maybe the cause could be if you have other chats open like recently they automatically made you join your country's /chat when you login and people talking within the chat could be shortening it (even though they are't visible in the main chat and this is just a guess)

No, I experienced it last night and I don't have a /chat (I'm on Linux, with my browser being Seamonkey). Both I, and Dracoleaf, are in America and we don't get one.
The issue is that the chat box itself shortens and does not have as many lines as it should. Not thast there's a chat taking up the space. Come on.