- Ensure Labs => Unread Message Icon feature enabled
- Install "TamperMonkey" chrome plugin
Bertrand Schneider for the code snippet and fix.
It looks like there was a regression that causes two favicon
link elements to be generated in the GMail markup, causing only the first to take precedence for the page (the default favicon) and the smart favicon with the unread count not to be shown.
https://mail.google.com/mail/u/0/*if your public GMail is your first logged in google account.