I have a strange issue that happens with both the Chrome and Safari browsers. It only happens when the browser is maximized on my 2nd monitor.
A rather large (toolbar sized) rectangle obscures the top-left of my window. It occupies about half the screens width and is a few textlines high. It is filled with junk or old screen contents. It does not scroll.
Hard to describe but here is a screenshot (I've added the red box).

So, is this a known problem? Anything I can do about it?