All foreground content, including scroll bars, should be kept within a safe area that has the same margin from the left and right edge of the screen in landscape. On that side which has the sensor housing the margin is calculated without the housing. Background content such as backgrounds in game menus, 3D worlds in games, etc, should extend all the way to the edges of the screen.
I've yet to see anything from Apple that would indicate how Safari for instance will render website background images and colors. I imagine it will extend them all the way to the edges of the screen, rather than limiting them within the safe content area.