This thin white bar appears every time you are swiping through screens, opening mission control and etc. It doesn't happen if you are using the keyboard to navigate through the pages but it does happen if you are scrolling through pages with the trackpad.
I would like add that I am experiencing a similar problem on an Intel based iMac. A white line appears at the top of the screen as soon as I start navigating the pages with the keyboard. It will disappear if I click the mouse, but will reappear when I start navigating again.
YACReader
