High resolution displays and GNOME

True. Though reference pixels aren't a great way of describing raster images either (you'd want real pixels really, or ideally of course vector everywhere), and border widths might be better served with something like "thin" / "medium" / "thick". You'd lose a certain amount of pixel perfect expressability, but in reality you do anyway when you deal with varying pixel densities.

> Unfortunately we're in a situation where a of code already uses the term "pixel", and we're keeping that API, just making it do something slightly different.

I see that, and your abstract pixel way is a pretty clever hack to make things work well with new screens. But I wonder whether we can imagine a better way to lay things out going for the future. I haven't actually coded GTK+ much at all, or Qt (Tk and CSS are the only graphics layout engines I know well), but perhaps there are already better systems available that I'm just ignorant of.