What's the physics solver used here? This demonstration quickly runs into common problems associated with overly simplistic solvers/integrators where everything starts to become unstable and "vibrate" / "twitch".

Bit twitchy indeed but still fun. Instantly took me back to "rubber glenz vectors" in classic Amiga prods[1]

[1] https://www.youtube.com/watch?v=OZTnR3FpUEA&start=413

It seems to be broken on Firefox. It shows 3/8 players, the circles are flickering madly and I can't drag them.

Because there are no instructions at all about what will happen when you click, I almost immediately created a "transporter accident" where I spawned in a new shape that intersected with an existing shape. They became locked together, and this new compound shape seems to exist as some kind of physics engine edge case. It struggled madly to find a low-energy state, rolling and jittering around, sometimes seeming to almost stop, and then gain energy again from nowhere and start moving again.

Wow, didn't expect to see my implementation here :)