It is now a toggle button on the Start Menu/Screen itself. Open the screen and look on the top right for arrow boxes pointing outwards (if you're on the Start Menu) or inwards (if on the Start Screen)
I believe that option has been removed as of 9926. The functionality has been replaced by allowing you to run the Start menu in full screen mode.
Click the Start menu, then click the button in the top right (the one with the 4 directional arrows on it).