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 being dragged;
draggable - the element being dragged;
droppable - the droppable element over which the draggable one is placed. This will be different from the node;
valid - whether the drop is allowed. Setting this to
false will disable the drop operation and will not add any valid drop indications such as highlighting or different style.