Etwas

Xephyr is a a kdrive server that outputs to a window on a pre-existing
'host' X display. Think Xnest but with support for modern extensions
like composite, damage and randr.

Unlike Xnest which is an X proxy, i.e.  limited to the
capabilities of the host X server, Xephyr is a real X server which
uses the host X server window as "framebuffer" via fast SHM XImages.

It also has support for 'visually' debugging what the server is
painting.