On Windows, I use AMD's VSR (Virtual Super Resolution) to set 5k screen resolution on a physical 4k display, letting me set the UI scale factor to 200% and have reasonable font sizes with clean icons (125% and 150% looks fuzzy). Under Linux, it is an xrandr spell that achieves the same thing.

How does this work? Don't you have to scale the virtual 5k resolution down to 4k? Wouldn't that also generate fuzziness?

If a stroke is 1px wide at 100%, 2px at 200% / 5k, how does that stay sharp when you scale it down to 80%?

Or is the idea that whatever algorithm AMD uses is better than the one Windows uses?

Say you have a 28" 4K display but you want to run 2560x1440 resolution to get you desired UI size. You can either run 2560x1440 scaled up to 4K, or 5120x2880 scaled down to 4K. The latter will look better.

Or you run at 4K and let applications scale – which sadly not all support properly.

This is what I usually do. I run 4k at "100%", but then I increase font size as needed. I usually don't care about the icons, widgets, etc since I rarely interact with them, so to me, it's actually a good thing they're small. This way everything is plenty sharp, and I also get more screen real estate.

The reason I use a 4k screen is to have sharp text, so I try to avoid scaling.

That’s why – despite being a free software advocate – I currently run Windows for all critical software, as on Windows I can scale content without re-scaling windows. (Qt and Android support it as well, but Gnome and macOS don’t).

I'm not using Gnome, but scaling the text works fine for GTK apps on I3.

Yes, but Elementary, Android, Windows and Qt can scale icons and all elements together with the text to any arbitrary scale, pixel-perfect, without any post-processing.

GTK can only scale text, or scale the whole window to 2x, 3x, etc and then scale it back down in post-processing.

Downscaling doesn't bother me as much for some reason. Icons scaled to 125% or 150% from their 1x size look more blurry than the 2x version of the same icon scaled down. It also looks better to me if the system uses integer scaling to blow up the 1x icon to 200% size first, then scale down from there.