1) Voice over say:
"to display a list of choices, press Control-Option-Space. But JCombobox doesnt expand on this short key. It expands on "Arrow down" key only.
2) Only Control-option-Arrow moves voiceover focus among items in combobox popup. Marked (selected) item on popup is not synchronized with VoiceOver's focused item in popup.

3)Only Control-Option-Space selects item, Enter and space do not select VoiceOver's selected item.

Tested on OS X 10.11.4

Not a regression.
Platform specific - yes, OS X.