Almost there, Arnaud.

I think that the header box needs to be big enough to show one line of each main header.

It could maybe resize itself but when Iget a message that has many recipients: I definitely shouldn't have to scroll to find the subject or sent date. In such cases, only one line of the To header should show by default.

Maybe the Headers frame, on top of message body, could extend in order to show everything at once. So I don't have to scroll down...