Obviously that cape normally hidden while wearing elytra. However elytra can be hidden with mods like Cosmetic Armor Reworked. So it would be nice if cape physics while flying were fixed.

Probably will not add anything explicit for flying. I still have on the todo to try to support stuff like crouching/swim mode changing the direction gravity points at (the main issue is getting how the player is rotated). That might also fix flying.