I have never seen this problem, but here is a recent similar post
with a suggestion of checking your graphics card / driver:
Wavy vertical lines distort top and bottom
when I use Scale to frame size in CS6
What display card are you using?
What driver version do you have installed?
I have a GeForce GTX 550 Ti
Lately I had updated to the latest driver 320.18. I decided this morning to
roll back drivers to the Nvidia 314.22.
I do have small black bars at the bottom and top of the screen but I don't care. The
video looks great on my external monitor.
It honestly looks like the graphics driver is redrawing the final line as filler and the size makes it look like where the task bar would be.
Glad to hear that rolling back the drivers helped - I've heard comments that the 320.18 should be avoided for now. Other people are reporting issues as well.
This bug should be resolved in NVIDIA's next release. In the meantime try rolling back to the previous driver version.