On Windows and Mac all works fine.

Run an application based on the attached FXML file: it defines a top level MenuButton which contains a SplitMenuButton and another MenuButton, among other things.

If you open the SplitMenuButton then as soon as you move the mouse cursor to select one of the "Inner Split Action" the list of values is dismissed.

When you click the InnerMenuButton the top MenuButton closes; if you press InnerMenuButton without releasing then you can hover one of the "Inner Action" values but none can be selected.

This issue hits SceneBuilder 2.0 (see DTL-6659) where a given number of Inspector editors are using a popup within a popup.