The code isn't public yet; I just saw the demo, and I believe it's rendering the same objects on the left and right with different camera settings. When the window moves, the x, y coordinates of the right object update in both renderings.
isn't this it? https://github.com/bgstaal/multipleWindow3dScene
Yup
Here play wit it https://bgstaal.github.io/multipleWindow3dScene/
Ok! TIL! And it doesn't even get confused by my two screen setup which makes me wonder about fingerprinting. I didn't know apps could look that far outside their own window.