once again, this is probably my own personal eccentricities rather 
than the expected behavior, so I have changed this behavior.  BTW, IMP 
4.0 marked the messages as read not because of any overt act by us - 
it was marked read due to sloppy programming (imap_body() calls in 
IMP_Contents:: set the seen flag without us expressly setting the flag 
no matter whether if we wanted the flag set or not).

This is really unintuitive and not how IMP 4.0 worked.

This is/was the desired behavior.  I was/am only considering a message 
'Seen' when a user actively views that message - not passively views 
that message via a multi-message/thread view (although multi-message 
could be considered 'active' viewing since the user had to specific 
include that message).

But in either case, the entire message may not be viewed - only the 
first viewable body part.  So marking the message seen may incorrectly 
tell the user he has 'seen' the entire message, when that actually 
isn't the case.  The only way we can guarantee to a user they have 
seen all viewable parts of the message (well... almost guarantee: see 
Bug 1863 and Bug 1866) is to access the message via the message screen.

Same for thread view btw.

The seen flag is not set if viewing several messages at once with the 
multi-message-view.