element need to be sticked to bottom (position:absolute) vs part of the DOM (position:relative)

I have a requirement where element need to be sticked to bottom (position:absolute) vs part of the DOM (position:relative)

UseCase “footer” stick to bottom (position:absolute) when elementA and elementB are not in view. “footer” position relative when elementA is not in view, elementB is in view. “footer” position relative when elementA, elementB both in view.

tried waypoint, offsets. having jittery flashing footer when elementB comes into view while scrolling. I do not think observer api is helpful here.