This bug applies to macOS only.

To reproduce:

- run the attached code sample
- click on the menu, select "Use Custom Menu Item"
- click on the menu, verify that a custom menu item (a button) appeared in the menu
- select "Use System Menu"

Expected:
- the menu bar stays where it was (inside the window) because custom menu items are not supported inside the system menu

Observed:
- the menu disappears
- stderr outputs "Warning: MenuBar ignored property useSystemMenuBar because menus contain CustomMenuItem"

Notice that if the order of steps is modified slightly, the menu bar does behave as expected:

- restart the sample application
- select "Use System Menu"
- unselect "Use System Menu" (in the system menu)
- select "Use Custom Menu Item"
- select "Use System Menu"
--> observe that the menu bar remains inside the window as expected.

 
stderr: MenuBarSkin:733