I've resorted to disabling scrolling in the collection view for a specific amount of time after a new cell is added, then removing the attachment behavior after that time has passed using its action
property, then adding a new attachment behavior again immediately.
That way I make sure the upwards animation stops before the collection view is scrolled left or right, but also the left/right bounciness is still there when scrolling.
Certainly not the most elegant solution but it works.