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.
"Mark as unread" is presented as a secondary action for those notifications that are read (check T114357 for more details on secondary actions).
For messages, marking as unread turns the notification to the same status it was before. The notification badge is updated but not highlighted in blue (since the notification is not new). As with any other unread notification, it can become read by opening the notification, marking it as read using the "X" or using the "mark all as read" action.
For alerts, marking as unread keeps the notification as unread even if the panel is opened/closed multiple times afterwards. Unread notifications will appear on top even if more recent notifications appear (similar to what happens in messages where unread bock is always at the top). A special close action is provided to (a) communicate that the notification has been "fixed" and (b) allow to mark it as read. Alerts get marked as read if they are opened (as described in T118722, this may change in the future if we decide to keep a stronger fixed/pinned state).
**Further aspects to observe**
After providing support for marking as unread, we may want to observe the user behaviour to determine the following:
- Should we provide also a secondary action for "mark as read" to keep the symmetry? We didn't considered as part of the initial ticket since there are already multiple ways to mark as read, but we want to check if users are expecting to find it also as a secondary action.
- Should we differentiate more the behaviour between alerts and messages? If we find that "marking as unread" is not enough to explain why alerts don't get unread automatically we may want to surface that more explicitly.
- Should marked as unread messages also become more "sticky"? Messages don't get automatically read when opening the panel, but they can become read when visiting their associated page. An explicitly marked as unread message becoming read when visiting the page may or may not be the expected behaviour.
This card tracks a proposal from the 2015 Community Wishlist Survey: https://meta.wikimedia.org/wiki/2015_Community_Wishlist_Survey
This proposal received 18 support votes, and was ranked #45 out of 107 proposals. https://meta.wikimedia.org/wiki/2015_Community_Wishlist_Survey/Notifications#Echo_notifications:_mark_to_read