Sorry i don't know what you mean. I can move your virtual windows here without seeing any artifacts or slices. Do you mean that when moving the upper half of the window is at another position (horizontally) than the lower half?

This might be because your graphics hardware is slower than mine. Can you give me your specs?

If it doesn't look good on your machine, i suggest you turn V-Sync on again and find another sollution (i can't think of any yet, if i do i'll let ya know).

You say your FPS increases when you covered your window. :? Does it restore quickly to a normal number? How do you calculate your FPS?

It might be an idea to render something that moves (e.g a bouncing rectangle). Cover and uncover your window again. Does the rectangle move faster than normal?? This is a way to determine whether the framerate has actually increased or you FPS-code is not working properly.

Hope it helps.