I've been experimenting with a Remarkable Paper Pro.

Additive inking works pretty well. As long as I am adding black ink to the page, everything is responsive. But if I want to push other updates asynchronously, or (god forbid) erase something, then there is the danger of clobbering the user's inkwork. I don't have any deep insights here, beyond waiting for human confirmation before updating anything.

I wonder if it is possible to detect when the pen tip has "left" the proximity of the page? If I knew this, I'd know when it was safe to update the UI.

I have decided not to use any scrolling at all. I am paginating everything and positioning input areas near the vertical middle of the device for comfort.