There's some performance issues on 10x10 and 15x15 boards (it becomes harder to paint or mark a square for some odd reason, like it is not reading the mouse input properly)
Please makes it so X-marked panels cannot be painted on by accident.
Please make it easier to paint rows and columns, like the picross games on 3DS does.
All in all, it's a very short and decently challenging for its length, could eventually expand this project into a retail game.