One of the new features in Nextcloud 12 is the contacts popup:
You can easily add it to your app with some little lines of javascript and maybe some CSS changes (make sure you have not overflow: hidden;
on the parent element.
You can find some sample patches:
- Activity
- Notifications
- Comments (beginning of the diff)
If you want to provide an action for the popup, you can have a look at:
Only integration into all the apps in both ways (providing features and displaying them) can make this feature as powerful as it can be.