Comment # 46 on bug 1089932 from Fabian Vogt

I think I can finally reproduce the issue. It happened while using nouveau on a
workstation, but not with the proprietary nvidia driver.

Please confirm that these two commands (in icewm, after xrdb -remove and the
-dpi line in sddm.conf commented out) produce the same output:

QT_AUTO_SCREEN_SCALE_FACTOR=1 QT_SCREEN_SCALE_FACTORS= sddm-greeter --test-mode
---theme /usr/share/sddm/themes/breeze-openSUSE
QT_FONT_DPI=96 QT_AUTO_SCREEN_SCALE_FACTOR=0 QT_SCREEN_SCALE_FACTORS=2
sddm-greeter --test-mode ---theme /usr/share/sddm/themes/breeze-openSUSE

And please confirm that this command produces a usable sddm scale:

QT_FONT_DPI=192 QT_AUTO_SCREEN_SCALE_FACTOR=1 sddm-greeter --test-mode ---theme
/usr/share/sddm/themes/breeze-openSUSE

If this is the case, the issue is that Qt sets the physical DPI according to
the physical size of the monitor, but keeps the logical DPI at 96 instead of
scaling it as well.

The code states that if the logical DPI isn't forced (either by QT_FONT_DPI or
Xft.dpi), logical DPI is set to the physical DPI. Apparently this is not the
case here.