When I use ExtendedBorders = false and I maximize the window, the buttons in the window title bar (Close, Maximize, Minimize, …) are not positioned correctly.
However, when I use ExtendedBorders = true then this problem doesn't appear.
I attached a picture to help you see where the problem comes from.
NOTE: I'm still using the fix I described here. But I verified and it has nothing to do with it.