There should be a done button right top. Tapping the back button always feels like the typed text will be deleted. It's not very intuitive imo!
also, when posting a comment, I HATE that you can swipe down to go back. I was typing very long comments twice and both times something got stuck, I swiped down to go back to the beginning of the text and instead it went back and completely deleted my comment. infuriating.