[Dovecot] Thunderbird Problem - What causes this?

[Marc Perkel]

Strange problem and I'm not sure what's causing it. I'm using IMAP. A 
new message arives in the inbox. I see it displayed in the message list 
in bold. I click on the message and it looks like it's reading it by the 
previous message that was in the window remains and the new message is 
still bold (unread). However I can then click on an old message and then 
click on the new message again and it reads.

I have also seen Thunderbird be in a somewhat unresponsive state. You 
can click on messages and they don't read. But Shutting down Thunderbird 
and restarting it make it responsive again.

Has anyone seen this?