Is it proper to submit an incomplete game, say one where a player can't actually win but can only manually quit once they're bored, when the deadline comes around? I'm pretty sure I'll be finished long before then, but I'm just wondering in general.
It's totally up to you. If you have something to showcase, even if it isn't finished, I'd say to go ahead and submit anyway! This jam is mostly about learning and getting inspired to do more gamedev, as opposed to a competitive jam, so you do you!
