droppableOver event is fired when a valid draggable target is placed over a TreeView node that can accept a drag (e.g. a non disabled one).
This event can be cancelled - in this case there will be no droppable hover state change for the node.
The following additional properties are set to the event object:
element - the node on which a valid item has been dropped;
draggable - the element being dropped;
droppable - the droppable target. This will be different from the node;
valid - whether the drop is a valid drop;
cancelled - cancels the movement of the dragged element from its original position. Always set to
skipAnimation - if set to true, the dragged element will not be animated back to its original position. Normally when the drop is valid, there will be no animation, while an invalid drop will set this to false and animate the dragged element back to its original position in order to show that the drop is not possible.