Comment # 58 on bug 1089932 from Fabian Vogt

The same bug also causes an issue with Qt on non-Plasma, if a monitor is >=
144dpi but Xft.dpi is not set, Qt draws the UI with a 2x scale (it tries to
avoid fractions when autoscaling) but the fonts are still 96dpi as X supplied.

So I chose to try a fix in Qt itself instead of a workaround in sddm.
If that works reliably, I can submit to SLE (deadline early tomorrow!).

Please test
https://download.opensuse.org/repositories/home:/Vogtinator:/branches:/openSUSE:/Leap:/15.0/standard.

Without any ServerArguments in /etc/sddm.conf or any other configuration
everything should work correctly OOTB.

(Make sure that Xft.dpi is not forced to 96 by your DE though)