Consider running attached code and take NativeMenuBar > Debug > Submenu: you enter an infinite loop.
The line of code where I do "menu.getItems().add(new MenuItem("A new menu item !"));" is wrapped into an if with a max value of 10.
If you change this value to 1 or 2 then you won't enter infinite loop; as soon as it is 3 or above you can never open one single time the submenu, you enter directly the infinite loop.

I suspect a side effect of the RT-19957.