Follow-up from discussion: Should we use a persistent cache (never falls out of cache), or a cache that can expire? This is related to whether we need a DB index on seen-ness.

In T110731#2306088, @Mattflaschen wrote:

Follow-up from discussion: Should we use a persistent cache (never falls out of cache), or a cache that can expire? This is related to whether we need a DB index on seen-ness.

I think the most straightforward approach would be to use MainStash, which is persistent.

What is the behavior that we want from the user's perspective?

The highlight provides a visual cue on which are the new notifications among those that are unread. When the badge becomes blue/red it indicates that there is something new, and when opening the notification panel a brief highlight allows to understand which of the unread notifications are new.

The highlight on the badge and the highlight on new notifications work in sync since they refer to the same items (the new notifications). So I'd not keep the badge or the notifications highlighted for later once the panel is open, even if some of the new notifications didn't fit there. Highlighting notifications at the bottom of the panel as they apear into the view without being new may create more confusion than help.

If all the new notifications do not fit in the panel, the badge should still change from blue to grey, and all the visible items will show the highlight. That will already communicate that there may be new items beyond the panel and you want to check "all notifications".

Despite the use of the terms "seen" and "unseen", we are not keeping as unseen the new notifications that the user may not actually see in the panel if she decides not to scroll. So I'd think of it less of a literal "seen" and more like the propagation of the badge highlight to communicate what that means regardless on whether all affected items enter the viewport or not.