Sometimes, roughly every 2-3 months, WhatsApp Web (web.whatsapp.com), that I access through Chrome or Chromium, displays a green banner at the top of the left column:
"Update available. Click to update WhatsApp"
Clicking triggers a page refresh, then WhatsApp operates normally (it doesn't require a new pairing with the device). Actually, it is operative while the banner is displayed too, i.e. before accepting the update, too.
This usually appears at the start of a session (right when I visit the page), but it has also happened mid-session.
I'm confused by this request: the page has already the permission to set (and, I believe, update) its cookies, and it doesn't run any additional software on my computer. I wouldn't think it needs to do anything client side. So what is it updating and why would it need my click?
Edit: some recent update events: 16 October 2018, 5 April 2019, 24 September 2019, 11 October 2019.