Since Feathers in Haxe based directly on OpenFL (but not Starling as it was in AS3), is it possible now to add Feathers controls to Starling Sprites and vice versa?
As far as I know, no : the haxe version of Feathers uses regular OpenFL Display while Starling uses Stage3D. You can use both at the same time (I do), but Feathers content is always gonna appear on top of Starling content.
One could port the as3 version of Feathers to haxe though
Yes, this is how it works. You basically add Feathers UI components to the OpenFL display list as an overlay above Starling, which is rendered below in Stage 3D.
Or port the Haxe/OpenFL version to Haxe/Starling. Starling display objects don’t have every API that Flash/OpenFL display objects have, but they’re very close. The hardest part of porting between the two, in my opinion, is switching between mouse/touch event systems, which is where Starling diverges more than in other areas. An advantage of basing it on the version already in Haxe is that you could potentially merge in new bug fixes and features from time to time.