Use display aspect ratio, not pixel for tracking.
People are used to thinking about the aspect ratio of displays and used to not thinking at all about that of pixels. Most of the modes are simplt 4:3 modes and the text area is the entire window. The only oddball here is the Commodore 64 and 128 40-column modes. Because they have a border around them that's wider on the sides than the top/bottom, the display aspect ratio is actually narrower than a normal NTSC screen (6:5). It seems the PAL version actually has square pixels, but nobody has asked for a PAL Commodore mode, and I think that has a different colour palette too so I'm not doing it. Just to frustrate DigitalMan a bit, the default custom aspect ratio is now 4:3 (but can be configured). At present, modifying the custom mode while *in* the custom modes "works", which no sane person would want when adjusting the aspect ratio.