How can I mark a thread as read in Lion Mail? In Mail 5.0, when I view a message, it's marked as read. No surprises there.

When I get a few messages in the same thread, and I view the thread, the latest message is marked as read, but I can't figure out how to mark the earlier messages as read. 

I've tried scrolling down to make sure Mail knows I've seen all the messages in the thread. I've also tried clicking the drop-down button to reveal all of the messages in the thread (in the middle "preview" column, or whatever it's called) and highlighting the message that's still marked as unread.

The only thing that seems to work is to right-click and select Mark &gt; As Read. 

Is that just a bug or am I doing something wrong?
In case you didn't know, when viewing the different messages in the right reading pane, if you select any of the individual messages they will be marked as read.  This gives you the ability to mark some as read and leaving others marked as unread.