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After pressing Alt+Down, choosing an item, and pressing Enter, the a11y focus won't go back to the combobox.

Windows:

Need to re-test the popup (perhaps with a FakeCombo) to ensure it doesn't read any nonsense (like "WinAccessible-ComboboxListViewSkin$8 10 List with 3 items"). This can depend on how our popups work for a11y in general, and also depend on ListView support.

Mac:

I didn't really manage to get it working there because some things just didn't work well on OS X 10.8. So it may need some more work. In particular, the fake relationship between the list and the combo can be troublesome on Mac, but IIRC, it's needed on Windows. 

Basic support added on Mac to report the date back to the OS.
On Windows it will must likely report the date as text.

MenuBar

Consider using AXMenuBarItem on Mac for MenuItem inside a MenuBar (as per Apple documentation). So far no benefits was seen from doing this to justify the extra complexity. Note the limitation of ContextMenu on Mac 10.8 are visible while testing MenuBar.

MenuItem / Menu
CheckMenuItem / RadioMenuItem / SeparatorMenuItem / CustomMenuItem

It is an AXMenuButton on Mac and a UIA_ButtonControlTypeId on Windows. On both platforms the fire action opens the menu.