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 — LabeledImpl</li>
<li>arrowButton — StackPane
<ul>
<li>arrow — StackPane</li>
</ul>
</li>
<li>context-menu — ContextMenu</li>
</ul>
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 — LabeledImpl</li>
<li>arrowButton — StackPane
<ul>
<li>arrow — StackPane</li>
</ul>
</li>
<li>context-menu — ContextMenu</li>
</ul>