JavaFX: Undecorated Window
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(); }}