If you type FULLSCREEN at the prompt you can override the windowed view.
Viewing post in The Troll and The Teddy jam comments
Right clicking in the game with a mouse or long pressing on a touchscreen will show you the full screen option.
Llewelyn, check 9E, 9F, 9G in this Itch upload guide to make sure your game launches in fullscreen mode:
https://itch.io/jam/cavejam/topic/543846/tutorial-uploading-your-adventuron-game-to-itch