Becasue X11 doesn't support mixed dpi monitors, so you are using a hack where it pretends to have the same dpi and downsample where necessary. For proper mixed

Becasue X11 doesn't support mixed dpi monitors, so you are using a hack where it pretends to have the same dpi and downsample where necessary. For proper mixed dpi under Linux, you must use Wayland. But then, under Wayland, the fractional scaling is still not done (the wayland apps will work fine at given fractional scale, but x11 app will be @1 upsampled, i.e. blurry. That includes the browsers).