Hm, IMAP provides a stable view of a mailbox (so that you can delete or move things by numeric index) until the next time you select a folder. In order to see changes to an inbox, you typically need to re-select it. I'm suspicious that the code above doesn't show signs of doing that.

I believe there's also a way to get push-notifications from the server, but I don't know the technical details there, just that Thunderbird+Dovecot make that work.