What fresh hell is this? After last month's purple bars that wouldn't go away, and the page jumping around which means I'm constantly opening a different thread from the one I clicked on on my phone (still happening btw), now this! I posted on a thread, and as soon as the page loaded I got screen-covering popup encouraging me to sign up to Mumsnet. Which can't be closed, the x does nothing. Had to start a new session to make it go away.
See!
It's especially silly because I clearly am signed up or I wouldn't be logged in and posting.