I've redesigned the Private Message system. Now instead of the shadowboxed popup windows, PMs will appear in full-screen windows, with a whole thread of related PMs all showing in sequence in the same screen.
This should get rid of a lot of the awkwardness of the workflow, especially on mobile, and remove the need for quoting the previous message when replying.
This rethink was a long time coming, and I don't know what I was thinking with the old system—it just wasn't very good. I'm hoping this is a big improvement. And I'm sorry for making everyone put up with the old system for so long!