DirectX Runtime Behavior
Various versions of the Microsoft DirectX runtime handle the following rotation situations on behalf of the driver:
The Microsoft DirectDraw runtime automatically fails any attempt to display an overlay while the display is rotated.
All versions of the DirectX runtime adjust the scan-line values that are returned while the primary surface is rotated so that the scan-line values cover the entire range up to the height of the resolution. Otherwise, an application that attempts beam chasing might stop responding if it waits for a scan-line value that is greater than the width of the display and that the application would otherwise never receive while in portrait mode.
All versions of the DirectX runtime handle all accesses to a rotated primary surface that are made by a windowed-mode device that uses various forms of emulation.