No popup notification for the second unread mail

Steps to reproduce:
- have geary opened in the background, all emails marked as read.
- receive one mail: everything is correct, sound, blue envelope, popup and unread mail incrementation (1) in Pantheon-dock.
- leave Geary unattended
- receive a second email: everything work except the popup.

The "leave unattended between the emails" part is important: once geary is opened, even without actually reading the emails (they are still marked unread in bold) it cleans the incrementation in pantheon-dock, the envelope turns back to white and the popup correctly shows on the next email.