Why does it says welcome back -user- click here to “refresh page”
TL;DR: This is a technical/programming question, not an off topic question about the Stack Exchange network. I'm interested in what purpose this serves as a security technicality.
This has intrigued me for a while. It is not a question about how to use StackExchange websites, or a support question - I'm interested into the specific technical reasons in why all StackExchange websites log me in after the page loads, requiring a refresh.
I'm talking about this message:
I can't understand why logging me in isn't done server-side, rather than as a message after the page has loaded. It's something I've never noticed on any other website.
Oddly, I don't notice this behaviour on stackoverflow.com - only other websites within the Stack Exchange network.