Comment # 23 on bug 1205585 from Fabian Vogt

The s390x tests in openQA do not use any kind of graphics emulation or hardware
at all, they perform the installation over VNC using Xvnc.

There are also tests for that on x86_64, which show the same issue. Compared to
s390x the "background" pixels are black instead of white, which is close enough
to the background for human eyes and openQA needles. This does not happen in
installs without VNC.

Good (1109): https://openqa.opensuse.org/tests/2865136#step/welcome/1
Bad (1110): https://openqa.opensuse.org/tests/2867407#step/welcome/1

The most likely cause is the fontconfig update in that snapshot:
  * Enable 10-sub-pixel-rgb.conf by default

My guess is that with Xvnc there's a different selection of X11 visuals
available and the installer selects one which does not quite work with the font
rendering method. It might be missing alpha for instance or somehow blend it
incorrectly.