In this discussion https://www.phpbb.com/community/viewtopic.php?f=461&t=2233736
Basically, extensions with front controllers should be able to use the core event core.viewonline_overwrite_location so when a visitor views the Who Is Online page, it can display the name of the extension page being viewed.
The issue is all front controllers are based on app.php
So if more than one extension tries to use this event for their own front controllers, one of the extensions will overwrite all other extensions trying to do the same.
So a user could be on a blog page, another on a rules page and another on a gallery page... But all three of those users may be listed as viewing the blog page (which is incorrect).
I would guess there would be a need for more than just app to be used here, perhaps the routing controller name, or app/blog and app/gallery in order to make each extensions front controller independent of each other.