title: How Stack overflow updates questions with new activity
body: <p>I was on a stack overflow page, without refreshing the page shows me that <strong>{number}</strong> questions with new activity, but if i check in Browser network panel, I can't see anything. How they keep on updating this count, without calling any background process.
  <a href="https://i.stack.imgur.com/DmSDo.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/DmSDo.png" alt="enter image description here"></a>
  Any idea how it happens.</p>