With JavaFX 8.0.0-b80, sometimes the popup appears just beneath the ChoiceBox (lower on the screen), sometimes on top of the ChoiceBox (overlapped).
The different placement of the popup will happen without changing the size of the selection list or any other aspect of the location of the ChoiceBox

In JavaFX 2.2.7 sometimes the selection list is rendered empty when it is not. That appears to be fixed in Lombard. The empty popup behaviour appears after calling getItems().setAll(some_non_empty_list); with a list that was larger than the previous available selections (a super set of them).