I think you will need a max-height at a more sane value there, like 1000 to 1500px.
So this feature will cover your initial problem of
and scales up to a certain point, but given Discourse native infinite scroll behavior, you don’t want the height for increase that much.
This feature won’t remove the “scroll withing scroll” anytime soon.
As for mobile composer, I’m looking into what we can do here.