To reproduce, compile and run the example below
- see the first item shown (and its toggle selected on opening the popup)
- click button to select an uncontained item
- expected and actual: value and selected item are sync'ed (verified by print out)
[ ignore JDK-8087555:
- expected uncontained items displayed
- actual: old selected item displayed
]
- open popup
- expected: no toggle selected
- actual: old toggle selected

Reason is a broken invariant (also partly noted - kind-of, a bit upside down - in JDK-8088012) in the implementation of choiceBox' selectionModel (maybe even in its super SingleSelectionModel?):

assertEquals(getItems().indexOf(selectedItem), getSelectedIndex())

Fix in ChoiceBoxSelectionModel, override select(item) to explicitly set selectedIndex to -1 if not contained:

    @Override
    public void select(T obj) {
        super.select(obj);
        // super doesn't updatee selectedIndex for uncontained selectedItem
        // if we don't add this, the incorrect radioItem is marked selected
        // with uncontained selectedItem (after there had been a contained once)
        if (obj != null && !choiceBox.getItems().contains(obj)) {
            setSelectedIndex(-1);
        }
    }

It might be cleaner to fix this in SingleSelectionModel (at least in the long run). But ComboBox has a super constraint violation specified as intended behaviour (aside: I think that's utterly impossible!), better not touch super for now.

The boundary between this issue and JDK-8087555 is that this is a failure in the selectionModel while the other is a failure in the skin


The example:

public class ChoiceBoxSelectedToggleBug extends Application {

    ObservableList<String> items = FXCollections.observableArrayList(
            "5-item", "4-item", "3-item", "2-item", "1-item");
    private String title;

    private Parent getContent() {
        ChoiceBox<String> box = new ChoiceBox<>(items);
        box.setValue(box.getItems().get(0));
        Button setValue = new Button("Set value to uncontained");
        setValue.setOnAction(e -> {
            box.setValue("myDummyValue");
            System.out.println("value/ selected item: " + box.getValue() +
                    box.getSelectionModel().getSelectedItem());
        });
        BorderPane pane = new BorderPane(box);
        pane.setBottom(new HBox(10, setValue));
        title = box.getClass().getSimpleName();
        return pane;
    }

    @Override
    public void start(Stage primaryStage) throws Exception {
        Scene scene = new Scene(getContent());
        primaryStage.setScene(scene);
        primaryStage.setTitle(System.getProperty("java.version") + title);
        primaryStage.show();
    }

    public static void main(String[] args) {
        launch();
    }
    
}