PASS IntersectionObserver in a single document with explicit root.
PASS First rAF
PASS document.scrollingElement.scrollTop = window.innerHeight.
PASS root.scrollTop = 150 with root scrolled into view.
PASS document.scrollingElement.scrollTop = 0.
PASS root.scrollTop = 0
PASS root.scrollTop = 150 with root scrolled out of view.