nice. Happy to share my thoughts here as well:
On mobile, the composer does fill most of the screen – but on desktop, one positive thing we heard about the in-sidebar composer was that it's helpful to see the context of one's feed, or a thread. We wanted to preserve that.
In terms of what happens when you "trigger another composer": For 5.0 – no changed behavior, really. We don't have drafts yet, but we want to. I have some explorations for when we get there.