verify that the IntersectionObserver() constructor does not defeat letterboxing protections
From #33534 (closed): As of Firefox 76, the
IntersectionObserver() constructor now accepts a Document object as its root, as well as an Element object. This lets you explicitly use a window's entire content area as the intersection bounds. We should verify that this does not allow someone to defeat the protections provided by letterboxing (probably it does not).
"[intersection-observer] Enable dom.IntersectionObserverExplicitDocumentRoot by default"