It seems to me that the display should reflect the chosen stereo mode, not the stereo mode reflecting the display.

But perhaps we have different use cases envisioned. I can't see how manually setting a 3D display mode through the settings/system/video is ever useful. I want to play a video and usually the video determines the stereo mode and the stereo mode determines the display mode.

Occasionally I may want to set a non-default stereo mode, like mono, but I can never imagine setting the display mode to something other than the current stereo mode.

Personally I would prefer to remove the explicit resolution forms with D3DPRESENTFLAG_MODE3DTB/ D3DPRESENTFLAG_MODE3DSBS, and make the base resolution (e.g. 1080p24) have flags for HSBS/FSBS/HTAB/FTAB capabilities.

Then SetNativeResolution would take the current stereo mode as a parameter which it can signal through the HDMI interface if that feature is supported.

You would need to make stereo mode distinguish between the half-SBS forms and frame-packed forms which would be used to determine pixel aspect ratio.