As a tech demo, its awesome, as a game, it's issues begin to show quite early. Firstly, there's not much reason to revisit the game other than getting a new high score, which for some is plenty of reason, but for others, it isn't enough incentive to keep playing, many roguelikes introduce random new assets into each run as a way of keeping the game fresh, perhaps elements of that idea could be implemented here to prevent the game from becoming stale. Another thing I disliked was the lack of communication, Wall Punch was one upgrade I didn't really understand, and since there's no real descriptions like in most roguelikes, I still don't quite understand what its supposed to do, it seems like it's meant to make it easier to resize the window by shooting its borders but I didn't really feel like it was doing that, and without a second guess I found myself rarely purchasing it, even a little description underneath the upgrade in the shop would be nice. Another thing I found was that many aspects of the game weren't too well thought out, one example of this is the homing upgrade, I would often try to shoot the border to expand in one direction but an enemy in the complete opposite direction of where I was firing would attract my bullets towards it and as I got more homing upgrades would eventually violently curve my bullets to the point I had to hug the wall I wanted to expand. This could've been solved by simply having a dot product check in place to see if an enemy is even loosely in the direction a bullet is moving, and having this get more lenient as you get better homing upgrades, but having it cap at 90 degrees left and right to prevent the violent 180 degree curving I talked about earlier. And since I spent so much time hugging the wall to expand it, it helped me notice another issue, since enemies rarely if ever spawn within the bounds of the main window's border, it led to many frustrating moments where I would get hit by something right offscreen when hugging the wall. Of course this wouldn't happen too often, and could probably be mitigated by solving issues that force me to hug the wall anyway, but it's a frustrating issue nonetheless, maybe there could be some sort of faint color-coded glow coming from the border of the window that indicates the direction an active enemy is in, with more "important" or objectively dangerous enemies having a stronger glow, and the laser star enemy having no glow, obviously. Speaking of that laser enemy, I noticed that it decreases spawn rates when one is active, this became a massive issue late game as the lack of enemies, and subsequently the lack of coins forced me into a monetary drought throughout the endgame, and made it too difficult to survive long enough to even get the money for a heal, much less find it with the growing price of shop rerolls draining my wallet before I could even purchase one. I could probably bring up many more issues but I think you get the idea, the gameplay is fairly flawed in many aspects, and since this was a game jam project, I'm not sure if you would be willing to update the game or if the intricacies of the gameplay loop's design were even that deeply considered, but its nonetheless an interesting project that shows off the idea of a multi-window and completely window based game quite nicely, I wonder if other games will do anything with this idea, maybe they already have.