Indeed, I didn't experience this anymore recently.

This would not be an issue with the session closing, since the message 
is marked seen before we close the session.

Given the changes in the IMAP library and the keyboard navigation code 
since this was reported, that may have resolved this issue.  If you 
can reliably reproduce, please reopen the ticket.

Can you try disabling closing session (disable session closing in 
IMP_Contents#_fetchData()) and see if that fixes this?

My guess this has something to do with the recent changes closing the 
session when loading the message data.  But I have yet to be able to 
reproduce.

This is what I get from the backend exemplary:

POST:
add        1
flags        ["\\seen"]
msgload        {7}SU5CT1g294760,294770
token        d_UfLRPcj3hQYB0_tbW0zSA
uid        {7}SU5CT1g294779
view        SU5CT1g

294760 had already been loaded in the previous request. Response:

imp:viewport/data:

{"294779":{"flag":["unseen","personal"],"mbox":"SU5CT1g","uid":294779,"size":"2 KB","date":"23.09.2012","from":"An: webmaster@tip4all.de","subject":"Neue Ergebnisse"},"294774":{"flag":["unseen","personal"],"mbox":"SU5CT1g","uid":294774,"size":"2 KB","date":"23.09.2012","from":"An: webmaster@tip4all.de","subject":"Neue 
Ergebnisse"}}

Next poll response:
{"294779":{"flag":["unseen","personal"],"mbox":"SU5CT1g","uid":294779,"size":"2 KB","date":"23.09.2012","from":"An: webmaster@tip4all.de","subject":"Neue 
Ergebnisse"}}

And the message is marked unseen again.

This is not deterministic, so it's really hard to track down or 
reproduce, but since recently (around the time when the temporary 
session close during message preparation was added, though this could 
be coincidence), sometimes when I navigate through unseen messages, 
the message that I navigated away from, is turned unseen again.