With regards to the CRT mask accurate this isn’t really an issue with the shader per se it’s to do with rec. 2020 and using HDR which effects all shaders in HDR. Admittedly HDR is a key part of the Megatron so it’s fair argument in one respect.
So does HDR/rec. 2020 break the masks? I’d argue it doesn’t in practical terms but technically you can argue it does. Certainly in person with the naked eye with your nose pressed up against the screen you can’t see the green subpixels being on in the red screen - the red screen is red and made up of faked red phosphors as far as you’re concerned.
This kind of makes sense as the red rec. 2020 sub pixels are trying to make the same colour as the red rec.709 pixels or indeed the red rec.601 phosphors which were both slightly greener - but you wouldn’t at the time say they were greeney red you would just say they were red.
The question Im trying to figure out is when I output pure red from my shader is the TV turning on those green sub pixels as its doing in mask accurate mode. Is this windows, the gpu driver or the TV converting that red into rec. 2020’s equivalent red. This may show us a bug somewhere which would be important to fix.