Right now this is only done on Wayland which has per-output scaling. X11 is limited to global scaling, and we don't do it there.
But if there's only one screen connected, global scaling is the same thing as per-output scaling, so we can safely doit there too for that use case.
Once the user connects a second screen with a different DPI, this will start to cause problems, of course. But that's already the case if the user manually sets a global scale on X11, since mixed-DPI multimonitor isn't supported on X11. So auto-generating a default scale factor for the single-screen use case on X11 won't result in a regression for the mixed-DPI multimonitor use case per se.
BUG: 422552 FIXED-IN: 5.27