OK, not an ad. Upon closer look, it's an email newsletter signup. Which pops up whenever your cursor leaves the page, or moves a significant amount on it. Like towards another tab, or towards a story link on the page itself. Once it pops up, it dims the rest of the page, and covers it. Clicking on links does not work until/unless you dismiss the popup.
I captured the form submission code for reference:
form action="//mc.us10.list-manage.com/subscribe/form-post?u=047b32e7f58e3630278c76a7f&id=9faf3094f5&popup=true" accept-charset="UTF-8" method="post" enctype="multipart/form-data" data-dojo-attach-point="formNode" novalidate=""
Firefox 52.9.0 ESR / macOS 10.14.0 (also tested in Safari, Chrome, Opera with same result)
Edit: it appears on the SR story page too. At least the page can be scrolled under it, and read around the banner. Personally, I try not to click on popups, because it can lead to unexpected results.