When using nested sub-menus the Navigation Collapsed item appears to break. When you click the first level sub item it appears to expand as it should, but the inner box is only equal to the height of the first level of sub items.
EG if you have two top level sub items, and two items in the first sub item. The box will not expand to show all 4 items, you will only see the first nested item. But if you leave the second level open, and close / re-open the first sub-level it is a little better but the second item from the top sub-level is hidden.
By default it’s also not very easy to determine which items are part of the nested sub-item. But that is something I should be able to take care of with custom css. It appears the javascript for this element is not calculating or applying the height values properly for nested menus.
You can see it in action here: https://oakhillag.bitstorm.pro/
(toggle is hidden on larger screens)
Opening both sub levels:

Closing and re-opening only the first level:

Layout of the example menu:
