The notification panel allows you to mark notifications as read in different ways (you can click on the notification to get to the content, click the "X" or the "mark all as read" action), but it does not allow to make a read notification to become unread again.

Allowing to mark read notifications as unread again makes sense to highlight there is pending work to be done (e.g., the user is planning to reply to a message) when the notification panel is used to organise the wiki work to be done.

Proposed solution
If we support the notification model described in T114357, we can add a "Mark as unread" action for those notifications that are read.

(Further design details to be decided...)

Additional considerations

• If we are providing a "mark as unread" for read items, should we provide the opposite action for read notifications? That symmetry may make sense but we need to consider how to better integrate with the existing mechanisms to mark items as read.
• This, as described, only makes sense for messages since alerts get read as soon as their panel is open. Alternatively, for alerts we could avoid those notifications explicitly marked as unread to become automatically read when the panel is open. In that case we may want to have additional considerations (e.g., mark them as pinned to clarify the difference in behaviour) which probably deserve its own ticket.