Some notifications aren't really about any page. Currently, those notifications are not associated with any page, and so they won't be displayed if you filter by a page using the (future) sidebar on Special:Notifications, no matter which page you filter by. This also means the number of notifications for each page does not add up to the total number of notifications for that wiki (although that can already happen when there are many pages, because we don't show more than 10).
However, we may wish to associate these notification types with users instead of pages. We could do this by associating them with the relevant user's user page, if it exists.
- `welcome`: not associated with any page
- `user-rights`: associated with the main page (yes, really! fixed by https://gerrit.wikimedia.org/r/#/c/293516/)
- `emailuser`: not associated with any page
What we could do:
- `welcome`: associate with the (welcomed) user's own user page
- `user-rights`: associate with (affected) user's own user page
- `emailuser`: associate with the sender's user page
However, this probably won't work too well, because the user's user page (or the sender's user page, in the `emailuser` case) may not exist. While we can associate notifications with nonexistent pages, we don't usually do that because it doesn't make much sense, and we are not able to filter by such pages. If we only associate with the user page if it exists, that may be confusing for the user, especially because their user page probably won't exist on wikis they don't frequent.
Perhaps we should figure out another way to list notifications that don't belong to any page? Or just leave this be?