I had this same problem, and I just figure it out, maybe it's your same situation
I could't move, resize the new message window, I was looking for an answer. I just found out, that if the Mail Window is Maximized, and then you try to write a message, this new window has the same maximized state and even there are no buttons at the top left corner of the widnow and indeed you can't resize this new message window, and even more, this window is at top and also you can't change activate the mail app window behind (This was very annoying to me) . So the easy solution was to set the Mail app window state in a normal way -not maximized-, Back to desktop, and that's it.