Was looking for a high quality video of the Sketchpad demo and ran across this:

I feel like everyone here has written this demo. It's one of the canonical graphics / physics / game tutorials. What am I missing?

Oh, I didn’t know these were common. I hadn’t run across anything like this and had wondered why someone hadn’t made a modern version. I thought Alan Kay said as much in one of his talks, but maybe I’m mistaken.

Unless I'm misunderstanding, it's just Soda Constructor-style iterative constraints, but without the features for animation, springs, etc.