You mean
this portal,
dot? I get it (and similar stuff) all the time, when "fastening" my cursor (in order to be able to scroll) to what I believe to be a blank part of this page. Apparently there are ads there which I cannot see. I had half a mind to post here in Suggs asking whether hidden ads are really legal, until it occured to me that different browsers (and different settings) will display different things. In other words, that ad may not be invisible to everyone. Sooooooooo annoying!!!
(Before anyone decides to enlighten me about the blessings of the scrollbar, I do know it's there... I just prefer the wheel.)