Though warning people that there is no recipient of their message is a good failsafe, I’m continually confused when I start a new PM and the username isn’t completed and then I realize it’s because I’m typing the title. Then after I type the title, I just tab on down to the message body.
I think the UX problem is that the initial focus is on the title, when what I’d expect from a couple decades of writing email is for the user/recipient to be the first thing to enter.
That’s because the recipient is usually pre-filled, e.g.
I click your avatar
I click Message
your username is already entered in the to: field
the focus is on the topic title
Why in the world would I need the focus on the recipients field at that point? Intent has already been demonstrated by action, e.g. “I clicked on a person” or “I am on someone’s user page that I wish to speak with”