TN panel weirdness with mask effects

With mask strength at 100% (CRT-Aperture), I get very noticeable TN panel inversion artifacts. When I open a screenshot from RA in GIMP and zoom to 100%, I get no artifacts. No artifacts in fullscreen mode in GIMP, either.

I tried RA in windowed mode and RA in full screen windowed mode; wasn’t any better. What could be causing this?

Really weird stuff!

All closeups taken of a screenshot open in GIMP:

Same thing in RA, artifacts clearly visible. Looks like badly scaled scanlines and some weird ghosting.

Starting to think this might be related to the video driver… does the driver still get used when running in windowed mode?