SetViewPort bug?

If you never clear that area it'll show any random stuff that is in that memory. Using DirectX the generic GDI rendering is completely overridden and won't come into play.

So if your viewport only covers the left part you need to at least clear that area on the right at least once (and probably everytime a lost device is being recovered).

how do you clear just that left over rect area then?

FillRect, or give your window a non-null hbrBackground parameter to RegisterClass.