Release notes entry for bsc#1089932 and related.

It's a rather complicated topic unfortunately, with no easy fix or workarounds
available.

Qt supports automatic per-monitor scaling on X. It uses the DPI value of the
virtual X screen to calculate the font size for the primary monitor, which is
by default 96. It uses the relative DPI of the primary monitor to derive font
DPI for all other monitors.

If the primary monitor is HiDPI (>= 144dpi), this means the font has
effectively a 0.5x scale on all monitors.

Applications which request it (e.g. VLC) have too small text on all monitors.

Applications which do not request scaling (e.g. YaST) are not affected and use
the same DPI value on all monitors.

Applications run on KDE Plasma or GNOME are not affected as Plasma disables
automatic scaling by default and GNOME sets Xft.dpi according to the monitor
scale.

Workarounds:

a) Use a non-HiDPI monitor as primary monitor. VLC is then scaled accordingly
on the HiDPI monitor.
b) Set the font DPI (Xft.dpi) using the DE configuration or "echo Xft.dpi:
<value> | xrdb -nocpp -merge" to equal the DPI of the primary monitor
c) Disable automatic scaling: export QT_AUTO_SCREEN_SCALE_FACTOR=0

(maybe this part into a separate section?)

HiDPI support in sddm:

To enable HiDPI support in sddm, edit /etc/sddm.conf and set:

[X11]
EnableHiDPI=true
ServerArguments=-nolisten tcp -dpi <dpi>

with <dpi> being 96*scale of the first monitor listed by "xrandr".