'native': an element can still scroll that way; the browser scrolls it.'contained': an element scrolls on that axis and is at its end, but its overscroll-behavior
is contain or none, so the wheel stops there and scrolls nothing.null: nothing inside takes it; root scrolls.
Who a wheel belongs to when it lands inside
root: a scrolling element between the target androottakes it, as the browser would give it to that element. The wheel scrolls across when Shift is held or its horizontal delta is the larger.