Isn't scaling from 3072x1728 to 3840x2160 (so 125%) very blurry? A one-pixel line at 3072x1728 would be spread over slight more than line pixel wide on 4k.

To get nice integer scaling one could render at 15360x8640 (16k) which is 5x3072x1728 and 4x3840x2160 but I doubt that's what Wayland is doing. No common graphics card could handle that.

The font hinting relies on the resolution too. By scaling the rendered pixmap 125% the fonts would look bad too. Rendering the fonts in a larger size at the native screen resolution should look better.

Who said anything about scaling rendered pixmaps? Proper UI scaling is handled at the desktop and UI toolkit level, instead of brute force changing desktop resolution or scaling rendered windows. That's why it's not a simple problem and support varies not only based on operating system, but also UI libraries used by applications, their combination, application developers updating their UI libraries and using the new APIs. Whether the UI toolkit exposes it to application developer by having the graphic api operate at pixel independent units or having the developer manually calculate sizes based on the scaling factor or relative to something like default font size varies. Basic geometric shapes and text (assuming vector instead of bitmap fonts) can be rendered at any size and resolution. As for more photo like images downscaling them by fractional amount typically works quite well, so if they are big enough for integer 2x scaling they also work with 1.25 scale.

Here is an explanation of fractional scaling of GNOME in Wayland which is what the parent is using at 125%:

https://discourse.gnome.org/t/how-fractional-scaling-works-i...

For scaling 125%, the client windows are rendered at 2x their size and sampled down. On the parent's system, a full-screen window would be rendered at 6144x3456 and scaled down to 3840x2160 (0.625%).

This gives a blurry result and is taxing on the graphics chip.

Qt apps and Firefox support scaling themselves which gives a much crisper result for fonts and line-art.

Isn't macOS using the same approach? I remember it even having a warning at certain scale that it will be more computationally intensive. And blurriness from downscaling nearest bigger integer size is nowhere near as bad as blurriness from software which use xwayland and gets rendered at 1x scale and then upscaled.

Is it really all that taxing? I mean, we're not talking about a 3D game here.

As soon as you open up a legacy X application it looks blurry. The main offender was Chrome/Electron but I think this has been fixed last year.