I agree; I couldn't think of a good way of dealing with that, because it's really down to how itch embeds iframes. I did see one game in the jam owner's testing video which displayed "please focus the game window" when it lost focus, which I think is an excellent idea and one I would have implemented myself if I had thought of it! I intend to for future jam games, certainly.
No shame in not getting to the end: Spacy Mazy is deliberately really hard. The only reason this is a shame is that nobody gets to see the cool end credits :-)