When I run this on my mac, I can see an empty window for a moment before it goes full screen. When I click on the orange rectangle to make it non-full screen, there is some brief flickering on the app window.


public class NonExitableFullScreenApp extends Application {
    @Override public void start(Stage stage) throws Exception {
        Rectangle r = new Rectangle(0, 0, 250, 250);
        r.setFill(Color.ORANGE);
        r.setStroke(Color.WHITE);
        r.setStrokeWidth(5);
        r.setArcHeight(20);
        r.setArcWidth(20);

        Scene scene = new Scene(new StackPane(r), 640, 480, Color.BLACK);
        stage.setScene(scene);
        stage.setFullScreenExitKeyCombination(KeyCombination.NO_MATCH);
        stage.setFullScreen(true);
        r.setOnMouseClicked(e -> stage.setFullScreen(false));
        stage.show();
    }

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