Allow users to explore the individual notifications contained in a bundle in order to allow users to triage and avoid being forced to mark all of them as read at once.
Bundled notifications provide a secondary "expand" action to allow users to access the individual items.
Bundles are groups of similar notifications. Thus, it does not make sense to have a bundle of just one item.
This also means that as the items inside a bundle are marked as read, the bundle will turn into a regular notification once there is only one bundle item left. In order to avoid confusion, a transition can help to visually communicate that the bundle is replaced by the internal notification. I illustrated the idea below:
Unseen notification highlight
In order to provide a subtle indication about the new notifications users get (to distinguish them from the ones that remain unread), new notifications become briefly highlighted as soon the panel gets open.
The bundle may get highlighted if it contains new notifications since the last time the panel was opened. However, the individual notifications inside the bundle should not become highlighted when the bundle becomes expanded since the distraction can be higher than the value the highlight provides.
For the specific case of cross-wiki notifications (T114350), a similar approach is followed to present multiple notifications. However, since cross-wiki notifications are very different to each other, several specific aspects are considered. So refer to the corresponding ticket for details on how the cross-wiki notification bundle work.