CentOS 7 - Updates Testing for x86_64: user interface/x: xorg-x11-server-Xephyr
xorg-x11-server-Xephyr - A nested server.
Xephyr is an X server which has been implemented as an ordinary
X application. It runs in a window just like other X applications,
but it is an X server itself in which you can run other software. It
is a very useful tool for developers who wish to test their
applications without running them on their real X server. Unlike
Xnest, Xephyr renders to an X image rather than relaying the
X protocol, and therefore supports the newer X extensions like
Render and Composite.