Question: How to prevent node from being dropped before another node at top level?

Is it possible to prevent node from being dropped before specific node at top level? This is possible for non-top level node since `nextParent` with latest state of children will be passed to `canDrop` callback. For top level node, `nextParent` is null and I can't figure way to get latest state of tree. Without latest state of tree, how can I tell in front of which node it's being dropped?

Example (How to prevent PF 1 and PF 2 from being dropped before SF 1 & SF 2?)

SF 1
--SF 1.1
----SF 1.1.1
SF 2
--SF 2.1
PF 1
PF 2