It took me a while to work out that Space and Shift switch the growth direction. But then the game froze as I clipped the corner at the end of the first corridor.

I suspect this is a symptom of how you've handled collisions. Always handle x and y planes separately. If you handle them simultaneously, then you can get funny things happening if a collision occurs on a convex corner.

Maybe just make "Space" toggle shrinking or growing?

I get to the cup and then I don't know what to do. Game crashes if I'm in contact with the wall and cup at the same time.

But I love the music.