Hi Devil,
Today I installed a fresh copy of the xfce image, did an update and dist-upgrade to check if the problem was in the desktop type (kde). The same problem with the side displays .... black, but mouse-pointer is visible.
jan@Z170:~$ inxi -SG
System:
Host: Z170 Kernel: 4.17.5-towo.1-siduction-amd64 x86_64 bits: 64
Desktop: Xfce 4.12.4 Distro: siduction 18.3.0 Patience - xfce - (201805132203)
Graphics:
Card-1: Intel HD Graphics 530 driver: i915 v: kernel
Display: x11 server: X.Org 1.20.0 driver: modesetting unloaded: fbdev,vesa
resolution: 1680x1050~60Hz, 1280x1024~60Hz, 1280x1024~60Hz
OpenGL: renderer: Mesa DRI Intel HD Graphics 530 (Skylake GT2) v: 4.5 Mesa 18.1.3