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I wanted to use Xnest to create a safe environment for selenium browser tests. But I can't start it:

> Xnest -geometry 1200x1000+0+0

Fatal server error:
Server is already active for display 0
        If this server is no longer running, remove /tmp/.X0-lock
        and start again.

Well, duh: My X server is running on display 0. Since Xnest needs a running X server to work at all, the error message seems kind of odd.

How do I start Xnest correctly?

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I missed the line in the documentation:

Xnest supports all standard options of the sample server implementation. For more details, please see Xserver(1). The following additional arguments are supported...

So the correct command line is:

Xnest :1 -geometry 1200x1000+0+0

To get programs on this display, either set DISPLAY to :1 or start them with -display :1

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