Active area in new message windows

When a message opens in its own window, the message header pane is active instead of the message body pane, so pressing the space bar to read further only expands (or closes) the message header. This is a less frequent need for most users and a less intuitive use of the space bar. Unless the reader notices the header pane change, it seems that the space bar has no effect.

When a message opens, make the message body bane active by default.