Really Nexys4DDR has 12-bit VGA output. That's now, of course as prototype. Image is pretty clear and color identical. Output produces 1920x1200 analog signal, but true is that up/down scalers damaging the picture.
Each Q in resolution excludes borders, forgot you?
This means that Nexys4DDR displays not working area in whole 1920x1200 - it can scale 320x200 and borders will be different in 16:10 - 16:9 - 4:3 etc.
When you connect your C64 to old 4:3 TV (really old) you'll see that visible border is more high.
When you connect to modern digital 1080p border will be less high. Of course current digital TV or LCD allows stretch, native, fill image formats. I mean border is nice handled in demos, but is not useful normally. Other situation is in case C128's VDC - it has no border and there can be problem, but Mega 65 has common with C128 only 64 mode, so it doesn't matter. Yes, I'm really 128er and sadly looking at M65 which maybe anytime will support C128 mode with its 2 desktops and 2 CPUs...
I mean solving resolution can be leave user to scale C64 or C65 image by device, but it solves not M65 new modes...
Miro