GPU Scaling always on for Windows desktop, but correctly off for other programs

Windows 10 1703 x64, AMD FirePro W5100.

Driver: Radeon Pro and AMD FirePro Software Version - 17.8 Beta

 

I have GPU Scaling set to off in Radeon/FirePro settings. Monitor native resolution is 3840×2160 and I usually have my Windows desktop set to that res.

 

The problem seems to be that GPU scaling is always used for the Windows desktop, ignoring my preference. However other programs which set exclusive full-screen mode don't have that problem; they are able to properly change display mode to e.g. 1920×1080 (and that's shown in my monitor's OSD).

 

If I change Windows desktop resolution (right-click desktop, Display settings, change resolution to one from the drop-down list), while the desktop changes appearance the video signal sent to the monitor is still 3840×2160. So it seems the AMD graphics driver always uses GPU Scaling for the Windows desktop. Though GPU scaling shows as off in Radeon/FirePro settings, if I change scaling mode (e.g. between Center and Preserve aspect ratio) the image on the monitor changes to reflect that. So I think that indicates a driver issue, rather than Windows being responsible.

 

After setting Windows desktop to 1920×1080 (but as mentioned it's scaled to 3840×2160), I can then run a program which sets full-screen 1920×1080 mode, and the video signal changes to really 1920×1080. On exiting the program, Windows desktop reappears at 1920×1080 but scaled to 3840×2160 again.

 

Has anyone else seen this issue? Is there maybe a registry key or some other way I can get the driver to always respect the GPU Scaling = Off setting?