[Dovecot] Thunderbird Problem - What causes this?

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?