How to drag and drop into tree's unvisible root element?

[desp]

Hello,
I have two 2-level trees and I need perform drag&drop operations between them. I need little customization - just drop only non-leaf items to root element (which is not visible) and set dropped item position where mouse pointer was.

[ettavolt]

You may want to set allowDrop and allowDrag data fields on nodes according to your logic. Works for invisible root too - just shows reorder icon.