Drop an Accordion a top level element.
Select either the Accordion of one the TitledPane it contains: in Insert menu the only enabled item is Insert > Containers > TitledPane, which is correct.
But now if you select an AnchorPane enclosed in a TitledPane of the Accordion 100% of the Insert sub-menu content is disabled ==> BUG, many items can be put in an AnchorPane.

There's most probably many cases like this one, which is quite confusing.