The message preview window used to scroll a page at a time by entering
a space character.  Once the bottom of the message was reached,
another space charater caused the selection of the next unread
message.

This behavior has been defined since forever.

Now, the only time the next unread message is selected is if the
current message doesn't overflow the preview window.

Also, the scroll a page at a time is broken.  If the message is more
than twice the vertical size of the preview window, the third space
character causes a scroll backwards!