TreeItem's leafProperty and setLeaf methods should be protected and not private. There is no way to inform the UI that the leaf property has changed because there is no way to change the leaf property's value.

Overriding isLeaf is insufficient to allow async code to run or to allow an update to the property. In other words, its a property, but there's no way for a programmer to change the value. Subclasses should be able to change the leaf's value directly just like all other properties in this class that affect the visual representation.