@ralf3u What resolution are you using on each computer? `xrandr --current` should give the resolution and how the monitor is connected.
What GPU and driver are you using on each computer? Use `lspci -v`, and look for "VGA compatible controller". The kernel module should also be listed there.
Since window managers do compositing, the mesa driver could be relevant: `glxinfo | grep -i opengl`. Also, see if the bug occurs if you run `LIBGL_ALWAYS_SOFTWARE=true geany`.