This test mimics an infinite scroll mechanism.
A "page" represented by a 5,000px high div is added. Using the IntersectionObserver we detect when pages become visible. When they do, they load a 20ms resource (so I can track server-side), then add a new page.
In this scenario, when Googlebot does its resize, the first page is visible, and a second page is added. A second resize happened, the new page was seen, a third page was added. And then it stopped.
This needs more testing, but it indicates that infinite scrolling may only partially work if using the IntersectionObserver.