I can't say that I've had that issue; but I understand. If you reduce the swivel down close to zero, it should be forbidden from doing that; drop it all the way to zero and you can constrain it to a flat plane.
It appears that the problem was the itch.io page trying to take control of my mouse.
The solution was to activate the full screen mode.
Browsers are a pain in the neck sometimes. I'm glad we got that figured out!
