Separate binder (element to bind onscroll) and target (element to look at bottom pixel)

Use case is when the overflow:auto element (binder) is already taller
(due to unrelated content) than the element (target) that holds the
collection items. (Imagine a 2 column layout).
You want it to load in more items as you scroll to the
bottom of the 'target' even if you're nowhere near the bottom of
'binder'.
Keep fetching more pages until the viewport is full, for the cases where loading 1 page isn't enough