The TreeTableView proof of concept I developed required a change to TableView.rowFactory. Rather than the following signature:
private ObjectProperty<Callback<TableView<S>, TableRow<S>>> rowFactory;

It should be the following:
private ObjectProperty<Callback<TableView<S>, ? extends TableRow<S>>> rowFactory;

Of course, if this change is made, it should be made to ListView and TreeView in the same way.