Thanks-- the escape key does this, but pressing the start button doesn't do anything from the main menu. I can see that it knows the button is pressed as it will change the input icons if a keyboard key is pressed beforehand. Can't seem to navigate to the button, either, if you navigate left using the analog stick or the dpad, it only moves the level selection to the left, but the menu items can't be selected via the gamepad. The start button will exit a level properly, though.
Okay, so I thought the problem still existed, even after reinstalling the game (through that option in the itch client), but I just uninstalled the game and installed it fresh, and it seems to be fixed! I was able to exit using the gamepad. Thank you, and keep up the excellent work. Really enjoying your game.