Well, actually you see less of the image. It happened here:
https://www.mumsnet.com/talk/am_i_being_unreasonable/5440810-gather-in-for-real-time-parking-wars
when this ad was the entire background. There was a parking diagram & when I clicked on it only the central part of the image was displayed; the 2 sides were still the ad instead of being the rest of the image plus a black banner. Sorry, didn't get a screenshot of that. The SS is what happened afterwards, when the ad took over MN & I couldn't see or do anything any more. I solved that by doing 'back'.
Chrome on a Chromebook.