To reproduce, run the example below
- open popup
- type some text into the field (or select by up/down from list)
- type right/left key
- expected: cursor moved one char right/left
- actual: cursor not moved

Was working as expected in fx8, is not working in fx10 (and probably not in fx9, didn't check).

In addition, ctrl-A has no effect (that is it's not possible to select all text in the editor) - which is not a regression, isn't working in fx8 as well as in fx10. This misbehaviour came up at SO https://stackoverflow.com/q/51943654/203657

public class ComboTextFieldNavigation extends Application {
    
    @Override
    public void start(Stage stage) {
        HBox root = new HBox();

        ObservableList<String> items = FXCollections.observableArrayList(
                "One", "Two", "Three", "Four", "Five", "Six",
                "Seven", "Eight", "Nine", "Ten");
        ComboBox<String> cb = new ComboBox<String>(items);
        cb.setEditable(true);
        root.getChildren().addAll(cb);

        Scene scene = new Scene(root);
        stage.setScene(scene);
        stage.show();
    }

    public static void main(String[] args) {
        launch();
    }
    
    @SuppressWarnings("unused")
    private static final Logger LOG = Logger
            .getLogger(ComboTextFieldNavigation.class.getName());
}