I agree with Bogey, it doesn't happen every time.
The only way I can describe it is thus.
(I'm on a MacBookPro using Safari, at the moment, FYI)
If I move my cursor to the very left, or right, hand 'white space', then the Starbucks ad arrives as a, sort-of, back-drop, behind the MN page. Then there's the 'roundy, circly' thing, which pops up on the centre of the screen. If you don't do anything, then the whole MN page 'sinks' to the bottom and you're stuck with a Starbucks page.
I've discovered the only way to circumvent it quickly, is by swiftly clicking on a thread title - any thread title.
It could be hard to take a screenshot, Becca, because the damn thing is constantly moving!