Refactoring in 4e1c058 was not correct since recipient_block
and message_content checked for if not condition while
recipient_header checked for if.
The naming of classes in 6077a33 was also not correct
semantically.
As part of this change, we port into the .messages class the work in
4e8e7348da to change overflow-y to auto,
not scroll (skipping that would result in a regression).