This is different, because "deleted" is an abstraction, and not 
necessarily the "Deleted" flag. A message could also be deleted by 
being in a trash folder.

Thank you for your comment.  However, going by just the flag 
explanation in my view is not sufficient.  There is no flag for 
'undelete', however, the right click menu will show 'Undelete' as an 
option.  By the same analogy, we could just have delete and repeated 
deletion would undelete a message.

The fact the Mark as -> Seen will result in changing an item from seen 
to unseen (basically doing the reverse)  is confusing.  Perhaps the 
work 'seen' can be change in the menu to 'unseen' depending on the 
current state of the message.

I understand if this is not accepted and no response is needed.  I 
just wanted to document my thinking here.   Thank you.

The current behavior is perfectly valid, because there is no Unseen 
flag in IMAP, just a Seen flag. So you are flipping the Seen flag on 
and off.

In IMP dynamic view, when only a single mail item is selected, the 
right click menu does not show 'Unmark as' as an option.  However, 
when two or more mail items are selected, the 'Unmark as' is available 
and you can then select 'Seen' to change a message from seen to 
unseen.  The only way to change a single selected message from seen to 
unseen is by right clicking, selecting 'Mark as'  and then 'Seen' to 
flip the status which is not intuitive.

Thank you.