Bug 1091103

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".