ComboBox does not move selection to matching prefix item on keypress.
If ComboBox is editable, you have to use a mouse click in order to type in the textfield.

   @Override public void start(Stage stage) throws Exception {
        HBox root = new HBox();
        root.setFillHeight(false);
        Scene scene = new Scene(root, 850, 550);
        stage.setScene(scene);
        stage.setTitle("Messages Sample");

        TextField textField = new TextField();
        ComboBox<String> comboBox = new ComboBox<>();
// comboBox.getItems().addAll("One", "Two", "Three");
        comboBox.setEditable(true);

        root.getChildren().addAll(textField, comboBox);

        stage.show();
    }