When there is a notification there is no way to mark all unread notifications as read (AKA Dismiss all) from the same user activity widget (or whatever you call it) see image below.
But if you go to https://meta.discourse.org/u/YOURUSERNAME/notifications this can be done from this scree. See below.
Can I request we have an option to do this from the same user activity widget box?
It’s just these notifications can get annoying and I don’t want my users to have to navigate around in order to turn them off and would rather they have a simpler solution.