There can be cases where when scrolling backward/forward the first/last page never gets visible in the viewport due to a large width/height of another page.
In that case the intersection observer would never trigger and thus, the chapter wouldn't get changed.
By using the Pager, which always takes up the full width/height, this problem will not be possible, since the start/end of the Pager always enters or leaves the viewport.
In the "vertical mode" intersection changes for the left or right side of the element do not matter.
Same for "horizontal mode" the other way around.
If not handled accordingly, it would cause unwanted chapter changes.
