I have multiple monitors on my Ubuntu system - 4, to be exact.
I'd like the fourth monitor to always display the same as the second. Thus, I tried this:
xrandr --output DP-0 --same-as HDMI-0
with DP-0 being the fourth monitor (the "mirror") and HDMI-0 the source I want to mirror.
This does work, DP-0 shows the same image as HDMI-0. However, fullscreen is broken. When I use Firefox, watch a Youtube video, and click fullscreen, the top Unity bar thing (displaying window title and the icons like Network, Clock, Shutdown on the right) does not disappear but stay in front of the Window. If I undo the xrandr thing, it works again.
How can I mirror one of my monitors to one other monitor, without affecting the other screens and without ruining fullscreen?