Fix two issues found while testing HTML side-by-side mode with a BBC
news article [1]:

1. When traversing the DOM tree to choose a scroll anchor, elements were
   skipped if their bounding rect (`element.getBoundingClientRect()`) did
   not intersect the viewport. This however did not account for content
   which overflows the element but is still visible to the user because it
   is not clipped.

2. The code used the intersection of the scroll root's bounding client rect and
   `(0, 0, window.innerWidth, window.innerHeight)` to get the viewport
   rect. In [1] the root element's bounding rect only covers the top
   (100vw, 100vh) pixels of the document, and most of the actual content
   of the page overflows that area. Resolve the issue by always using
   `(0, 0, window.innerWidth, window.innerHeight)` as the viewport,
   except when explicitly overridden (eg. in tests)

[1] https://www.bbc.co.uk/news/world-europe-60693166