I don’t get your motivation for preliminary calling the start()-method setting a stage as undecorated, but the following piece of code should do what you want to achieve.
package decorationtest;
import javafx.application.Application;
import javafx.stage.StageStyle;
import javafx.scene.Group;
import javafx.scene.Scene;
import javafx.stage.Stage;
public class DecorationTest extends Application {
public static void main(String[] args) {
Application.launch(args);
}
@Override
public void start(Stage primaryStage) {
primaryStage.initStyle(StageStyle.UNDECORATED);
Group root = new Group();
Scene scene = new Scene(root, 100, 100);
primaryStage.setScene(scene);
primaryStage.show();
}
}