Since this is a feature that would be useful to more than just Kaza (and could be done neater from the backend, with user settings and all, rather than being persisted only in the window), I’d suggest asking Discourse Meta about it.
If it gets implemented upstream, we’ll get it, and so will the steadily growing number of other forums running Discourse.
There are several problems with these lists (popular on certain phpBB forums), that mean they can never really be accurate, or implemented in a way that holds up to any reasonable standards.
Web applications receive a request from the user, respond to it, and close the connection. Accurately tracking user activity would require continuously sending activity notices to the server saying “this user is still here!”.
This would make the site slightly slower for each user browsing it, since it would have to continuously process these little pings. But more importantly, it’d be a waste of the user’s battery and bandwidth - two far too precious resources to be wasted carelessly.
If we rule out the option of sending dedicated data for presence notifications, there’s still another option: judge based on the data that is already sent - Discourse keeps track of how far into a thread you’ve read, you may post something, etc. This would open up another issue: Accuracy.
The user may still be on the site without generating any footprints, perhaps reading a long post, or pondering their next response. They may even be on a mobile device and lose connection. These would all cause them to appear and disappear off the active users list.
The solution here would be to interpolate, and count anyone who was active in the last x minutes. But this wouldn’t be accurate at all, and by reducing jitter in the list you’re also reducing accuracy even further, all in the name of usability… of a feature that wouldn’t serve any purpose if it wasn’t accurate. It also wouldn’t at all account for those following threads using email notifications (like me).
Also, a feature like this would require some way for people to opt out of being tracked - phpBB does this on the login screen, but it can get away with this because it’s bloody awful at keeping people logged in, whereas Discourse will keep people logged in for all eternity unless they log out themselves.
Aside from being a blatant privacy violation, showing up on an online list with no way to go invisible can put pressure on you to participate in threads you may not have a fully thought out answer to, or simply log out and lurk that way (thus opting out of bookmarks, likes, replies, protected forums, etc). Neither of these are things we want to happen here.
Finally, a forum is an asynchronous medium. It’s not a chatroom. You make a post, other people read it, whenever it’s convenient for them. They may be at work, or be asleep, or perhaps in another timezone. Maybe even all three at once, you never know. They may then choose to reply immediately, take their time carefully wording their response, or not reply at all. Someone may come around months after the fact and have a brilliant contribution to the topic.
As such, I personally feel like an online member list would be not only technically infeasible, but also straight against the spirit of a forum, and potentially detrimental to the community to boot.