A generic solution may be possible. Every flash.events.EventDispatcher in an app will dispatch Event.DEACTIVATE when Flash Player loses focus (and Event.ACTIVATE when it gains focus again). Assuming that you're using the Feathers FocusManager, its focus property may not be null yet when Event.DEACTIVATE is dispatched. That will allow you to save the focused object in a variable. When Event.ACTIVATE is dispatched, restore the focus by setting FocusManager.focus to the saved object.
Starling.current.nativeStage may be a good place to listen for Event.ACTIVATE and Event.DEACTIVATE.
If FocusManager.focus is null on Event.DEACTIVATE, then there may not be a good way to do it.