Possible functions:

• Back to top scroll
• Join community
• Create post

(PWA needs 'Back' on iOS)

Imho these tend to be more trouble than they are worth.

And the Leave community button is already far more visible than I would prefer - every time I write a reply to a post and click the Comment button I have slight anxiety that I will accidentally click the button to Leave the community instead!:-)

I think this is a good priority - finding these buttons on mobile can be a long scroll, and a genuine challenge to find if you don't know where to look. I have looked in the burger menu for the "join community' button more than once.

If it should be floating is another question. I don't particularly like floating bits. But the current position is a bit awkward.