Scan lines will never look correct in 1080, because it isn't an integer multiple of 240. They will always be uneven.
720 is however an integer multiple, so scan lines can be every third line.
Until we get higher res screens and software to support them, 720p is the best way to use emulated scaliness.
Hmmm... so that's why... Would it be possible to not fill the whole screen/use a border at 1080p to have proper scaliness at that resolution?
Just asking for XRGB purposes.