Comment # 19 on bug 1083873 from Hans Petter Jansson

(In reply to Ludwig Nussel from comment #17)
> So how to proceed here? Fabian, do you have an upstream bug reference to the
> changed sddm behavior? How does gdm decide whether to use hidpy or not?

The GDM greeter runs gnome-shell, and the formula used by gnome-shell starts at

https://gitlab.gnome.org/GNOME/mutter/blob/817a76a7f52e4910bd0f28b1fa183513645d255e/src/backends/meta-monitor.c#L1437

For HiDPI scaling to be enabled, the following must be the case:

- Monitor must be at least 1200 pixels tall (inclusive).
- If connected via HDMI, it must be at least 3656 pixels wide (presumably a
TV).
- Monitor's physical dimensions must pass simple checks for reliability.
- Both horizontal and vertical DPI must evaluate to greater than 192.

If the tests pass, the scale is doubled.

The scaling is also configurable within the constraints at the top of that file
(1x-4x in .25 increments, resulting logical resolution must not be too small).

There's additional logic for multi-monitor setups... I think on X11, they all
have to use the same scaling factor, and HiDPI monitors win.

The scaling factor is made available to the entire desktop. I'm not 100% sure,
but I don't think GDM adds any logic on top of this.