run this code and set focus to the combobox and click space - not effects.
 @Override
  public void start(Stage primaryStage) {
        primaryStage.setTitle("Hello World!");
        Button btn = new Button();
        btn.setText("Say 'Hello World'");
        btn.setOnAction(new EventHandler<ActionEvent>() {

            @Override
            public void handle(ActionEvent event) {
                System.out.println("Hello World!");
            }
        });
        
        ComboBox box = new ComboBox<>();
        box.getItems().setAll("First", "Second");
        VBox root = new VBox();
        root.getChildren().addAll(btn, box);
        primaryStage.setScene(new Scene(root, 300, 250));
        primaryStage.show();
    }