Very fun game! Incredible art, UI, and sound design! The story and dialogue were adorable as well. Personally, I didn't have problems with the lack of jump control midair, I thought it was a mechanic similar to Jump King. The backtracking was a nice touch as well, it didn't feel forced or boring. I felt like I was playing a fully polished steam release while playing this! I finished the game 100% in about 22 minutes, but I'm coming back to speedrun it >:)
My main issue was that certain jumps seemed to require very precise timing with the flap, namely in the part where you talk with a cube after stomping (I can send a screenshot if you would like). I also had a few glitches (I spawned underneath the platform in level 2 once?) and some lag spikes (which probably have more to do with my computer/web browser than with the game). These are all very minor issues however!
As a side note, this reminds me very much of my first GWJ game, which was also a precision platformer/rage game about a bird climbing up a tall level–but it was nowhere near as polished as this!
