ADDITIONAL SYSTEM INFORMATION :
Ubuntu 18.04.1 LTS, 64-bit

A DESCRIPTION OF THE PROBLEM :
Briefly, the Stage is displayed in the top left and then jumps to the center of the screen. This happens even if I set X and Y prior to showing the Stage.

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
Run the attached Application

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
The Stage is displayed in the center with no jumps.
ACTUAL -
The Stage is displayed in the top left then jumps in the center

---------- BEGIN SOURCE ----------
public class HelloWorldApp extends Application {

    @Override
    public void start(Stage stage) throws Exception {

        Button b = new Button("Press");

        VBox vbox = new VBox( b );

        Scene scene = new Scene( vbox, 568, 320);

        stage.setScene( scene );
        stage.show();
    }

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

CUSTOMER SUBMITTED WORKAROUND :
None. I tried setting the X and Y prior to showing the Stage. I also tried setting X and Y in the onShown handler.