Until Slack releases a new version with an updated Electron, you can use this workaround that worked for me:
sudo touch /usr/lib/libunity-electron_notification_fix See the full details in: https://github.com/electron/electron/issues/465#issuecomment-206582829 -- You received this bug notification because you are a member of Ubuntu Bugs, which is subscribed to Ubuntu. https://bugs.launchpad.net/bugs/1547448 Title: Ubuntu 16.04: Notifications of some apps displayed in a classic window To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu/+source/unity/+bug/1547448/+subscriptions -- ubuntu-bugs mailing list ubuntu-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs