xnest on testing doesn't start session

["Todd A. Jacobs" <nospam@codegnome.org>]

I'm looking for some tips on how to debug xnest on Debian testing. I've
been trying to test out some alternative window managers under the
current X session using xnest and xephyr, but regardless of which window
manager I choose all I get is an immediate return to the nested login
screen. What can I do to debug this issue?

If it matters, I'm using the nvidia driver packages from testing, rather
than custom-compiled ones.

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