There is a lot of code related to two-level focus. It isn't being maintained, and generally isn't in the best condition. We should investigate whether we can simply remove this functionality, as it does not seem to be something that is a priority in JavaFX at present (and nor for the foreseeable future).

This would mean removing the ConditionalFeature.TWO_LEVEL_FOCUS enum value, as well as all TwoLevelFocus* classes in the controls module.