When the intersection root is a Document, we use the viewport itself as
the root intersection rectangle. However, we should only use the size of
the viewport and strip away the current scroll offset.
This is important, as intersections are computed using viewport-relative
element rects, so we're already in a coordinate system where (0, 0) is
the top left of the scrolled viewport.
This fixes an issue where IntersectionObservers would fire at entirely
wrong scroll offsets. :^)
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