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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?

  • Mirroring limits the available resolution. That's probably the issue. – user692175 Nov 14 '17 at 19:27
  • All of my monitors run on 1920x1080. That should not be the problem. – Florian Bach Nov 14 '17 at 21:09
  • Not really a solution to this problem but updating from Ubuntu 17.04 to Ubuntu 17.10 solved this due to the new desktop thing (Gnome instead of Unity) which doesn't have this top bar on non-primary screens, so that xrandr command to mirror the screen does now work fine. – Florian Bach Dec 11 '17 at 18:10

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