App.java:
import javafx.application.Application;
import javafx.scene.Scene;
import javafx.scene.layout.Pane;
import javafx.stage.Stage;
public class App extends Application {
    public static void main(String[] args) {
        launch(args);
    }

    @Override
    public void start(Stage stage) throws Exception {
        Scene scene = new Scene(new Pane());
        stage.setTitle("App");
        stage.setScene(scene);
        stage.setWidth(100);
        stage.setHeight(100);
        stage.setX(0);
        stage.setY(0);
        stage.show();
    }
}
Commands:
javac --module-path .../javafx-sdk/lib --add-modules=javafx.graphics App.java
java --module-path .../javafx-sdk/lib --add-modules=javafx.graphics -classpath `pwd` App

Observed behavior:
When executed with the commands above from Mac OS terminal, the UI is "blinking", that is disappears and reappears again. See attached video.
Additional this is printed into the output:
Mar 23, 2022 2:08:55 PM com.sun.glass.ui.mac.MacApplication lambda$waitForReactivation$6
WARNING: Timeout while waiting for app reactivation