Looks like live updates for alerts is built into XF2.1 but how do you change the frequency? Appears to be a big lag and members are refreshing - previously we used an add-on which updated the alerts even if members didn't refresh fairly quickly.
It works the same as it always did in XF1 - it isn't live polling for alerts, alerts just come in when the page you're viewing performs an AJAX request.
You will still need an add-on for some sort of live alert.
This small addon will automatically refresh your online page if it is open in a tab. This is something from back in the old days of forums, and I thought it would be nice to have it available on XenForo. There are two settings for the addon...
Polls the server for new content, and adds them to the existing list of posts/messages. Updates alert & conversation totals along the way. This add-on tries to only use a single poller across multiple tabs for the same thread/conversation, and a...
For me it looked like it refreshes your tabs when you are online.
I didn't think anyone would leave their onlne user page in a tab to be refreshed. My mistake.