Below are two failing tests:
when listening to selectedIndex the item is null: expected is the external item
when listening to selectedItem, there are two notifications fired, the first notifying a null vs. the expected external item

The technical reason is the internal invalidationListener registered (by MultipleSelectionModelBase, similar though untested in SingleSelectionModel) to selectedIndex that auto-updates the item - when updating by -1 there's no item found, causing the null. Further processing sets the real uncontained.

There's a principal issue with correlated properties, though (see http://stackoverflow.com/q/27186755/203657): currently no way to defere notification of a property until internal state update of the bean is complete. Could be addressed ad-hoc in SelectionModel (custom properties, that extending classes can block in their firing) or by general support in core classes, as done f.i. in ReactFX.

    @Test
    public void testSyncItemToIndex() {
        Object uncontained = "uncontained";
        // prepare state, single select
        int start = 3;
        getSelectionModel().select(start);
        ChangeListener l = (p, old, value) -> assertEquals(uncontained,
                getSelectionModel().getSelectedItem());
        getSelectionModel().selectedIndexProperty().addListener(l);
        getSelectionModel().select(uncontained);
    }
  
    @Test
    public void testSyncItemNotificationCount() {
        Object uncontained = "uncontained";
        // prepare state, single select
        int start = 3;
        getSelectionModel().select(start);
        List values = new ArrayList();
        ChangeListener l = (p, old, value) -> values.add(value);
        getSelectionModel().selectedItemProperty().addListener(l);
        getSelectionModel().select(uncontained);
        assertEquals("expected single event", 1, values.size());
        assertEquals("expected newvalue is uncontained", uncontained, values.get(0));
        
    }