How to access Stage from a Controllers Initialize - https://www.youtube.com/watch?v=BHLDfI30WWw&lc=Ugxam2ZTD-Vouf68Pf94AaABAg

controller.java

package sample;


import javafx.fxml.FXML;
import javafx.fxml.Initializable;
import javafx.scene.control.Button;
import javafx.scene.input.MouseEvent;
import javafx.scene.layout.AnchorPane;
import javafx.scene.shape.Polygon;
import javafx.stage.Stage;

import java.net.URL;
import java.util.ResourceBundle;

public class Controller implements Initializable {

    private double x = 0,y = 0;

    private Stage stage;
    public void setStage(Stage stage){
        this.stage = stage;
    }

    @FXML
    private Polygon polygon;

    @FXML
    void close(MouseEvent event) {
        Stage stage = (Stage) ((Button)event.getSource()).getScene().getWindow();
        stage.close();
    }

    @FXML
    private AnchorPane topBar;


    @Override
    public void initialize(URL url, ResourceBundle resourceBundle) {
        topBar.setOnMousePressed(mouseEvent -> {
            x = mouseEvent.getSceneX();
            y = mouseEvent.getSceneY();
        });

        topBar.setOnMouseDragged(mouseEvent -> {
            stage.setX(mouseEvent.getScreenX() - x);
            stage.setY(mouseEvent.getScreenY() - y);
        });
    }
}

cSS.css

/*
#303139 - Background
#323339 - TopBar
#F75590 - Close
 */

.background{
    -fx-background-color: #303139;
    -fx-border-color: #303139;
}

.topBar{
    -fx-background-color: #323339;
}

.button{
    -fx-background-color: #303139;
}

.button:hover{
    -fx-background-color:#F75590;
}

Main.java

package sample;

import javafx.application.Application;
import javafx.fxml.FXMLLoader;
import javafx.scene.Parent;
import javafx.scene.Scene;
import javafx.stage.Stage;
import javafx.stage.StageStyle;

public class Main extends Application {


    @Override
    public void start(Stage primaryStage) throws Exception{
        FXMLLoader loader = new FXMLLoader(getClass().getResource("sample.fxml"));
        Parent root = loader.load();
        Controller controller = loader.getController();
        primaryStage.setTitle("Hello World");
        primaryStage.initStyle(StageStyle.UNDECORATED);

        primaryStage.setScene(new Scene(root, 600, 400));
        controller.setStage(primaryStage);
        primaryStage.show();
    }


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

sample.fxml

<?xml version="1.0" encoding="UTF-8"?>

<?import javafx.geometry.Insets?>
<?import javafx.scene.control.Button?>
<?import javafx.scene.layout.AnchorPane?>
<?import javafx.scene.layout.HBox?>
<?import javafx.scene.text.Font?>

<AnchorPane maxHeight="-Infinity" maxWidth="-Infinity" minHeight="-Infinity" minWidth="-Infinity" prefHeight="400.0" prefWidth="600.0" styleClass="background" stylesheets="@CSS.css" xmlns="http://javafx.com/javafx/15.0.1" xmlns:fx="http://javafx.com/fxml/1" fx:controller="sample.Controller">
   <children>
      <AnchorPane fx:id="topBar" prefHeight="35.0" prefWidth="600.0">
         <children>
            <HBox alignment="CENTER_RIGHT" prefHeight="35.0" prefWidth="600.0" styleClass="topBar" stylesheets="@CSS.css">
               <children>
                  <Button mnemonicParsing="false" onMouseClicked="#close" stylesheets="@CSS.css" text="X" textFill="WHITE">
                     <font>
                        <Font size="13.0" />
                     </font>
                  </Button>
               </children>
               <padding>
                  <Insets bottom="5.0" left="5.0" right="5.0" top="5.0" />
               </padding>
            </HBox>
         </children>
      </AnchorPane>
   </children>
</AnchorPane>