If you call setX or setY before stage.show() then it doesn't make any influence. If you call after then it make.

import javafx.application.Application;
import javafx.scene.Scene;
import javafx.scene.control.TextField;
import javafx.scene.layout.Pane;
import javafx.stage.Stage;

public class Main extends Application {
    public void start(Stage stage) {
        TextField text = new TextField();

        Pane pane = new Pane();
        pane.getChildren().addAll(text);

        stage.setX(1500); // comment me. this call doesn't make effect
        stage.setScene(new Scene(pane));
        stage.show();
        stage.setX(1500); //or comment me. this call makes effect
    }

    public static void main(String[] args) {
        Application.launch(Main.class, args);
    }
}