Tree inherits from the List class...
so you have itemRollOver and itemRollOut events which lets
you know the index of the item
and a selectable property (at the level of the list (or tree
in this case), not for a specific cell or index)
so, you could listen for itemRollOver and set the selectable
property of the Tree based on whether the index that you rolled
over is supposed to be selectable or not.