A DESCRIPTION OF THE PROBLEM :
The CSS Reference is missing the substructure for MenuButton
https://openjfx.io/javadoc/13/javafx.graphics/javafx/scene/doc-files/cssref.html#menubutton

Substructure

* label -- LabeledImpl
* arrowButton -- StackPane
  ** arrow -- StackPane
* popup -- ContextMenu

    <h4>Substructure</h4>
    <ul>
      <li>label &mdash; LabeledImpl</li>
      <li>arrowButton &mdash; StackPane
        <ul>
          <li>arrow &mdash; StackPane</li>
        </ul>
      </li>
      <li>context-menu &mdash; ContextMenu</li>
    </ul>