Its a popup ad that is appearing within a minute or so after accessing the site (could be an article on the main page, or this particular sub-form). The ad flashes up within a second and then appears to go to the page top, but if you are not at the page top there is no way to get there as the popup ad has taken over the entire page focus - and the only way to regain focus is to close the ad.
Since the page is effectively and literally "locked", there is no way to access the ad and close it; thus regaining control over the page. I've tried typing this up several times within the post editor, only to have the popup ad appear. The only way to fix this loop is to close the browser tab, and thus leave AC. Experienced the same problem at iMore before encountering it here on AC.
Had to type this in Notepad, rush to reply, and then submit the reply before I got hosed again.
Is anyone else experiencing this? I have the latest Chrome update (Windows 10).