Having the example:

public class HelloWorld extends Application{
    @Override
    public void start(Stage stage) throws Exception {
        VBox root = new VBox();
        root.getChildren().add(new Text("HELLO"));
        stage.setScene(new Scene(root, 200, 200));
        stage.setResizable(false);
        stage.show();
    }

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

}

does not hinder the Window to be resiable on Ubuntu Linux.