Hi. I see the co-op modes (in the "Single Player & Coop" tab) all require steam overlay to invite you friends. I only vaguely looked at the code and it seems you're using steam to establish a p2p connection to play. Is there some technical difficulty that doesn't let Zero-K use a direct IP connection? E.g. if you're physically in the same room it would make sense to just connect to each other over the wire without any steam intermediaries.
