selected piece get to the front

app/src/main/java/school_project/GameUI.java

@@ -50,6 +50,7 @@ public class GameUI extends Group{
                 piece_pos_click.y = (int) event.getY();
             });
             _piece.setOnMouseDragged(event -> {
+                _piece.toFront();
                 _piece.setLayoutX(event.getSceneX() - piece_pos_click.x);
                 _piece.setLayoutY(event.getSceneY() - piece_pos_click.y);
             });