You can trick a Wayland compositor to run as the child of a compositor you're not currently in by overriding the WAYLAND_DISPLAY variable.

Using this, get the toy compositor to run inside the toy compositor.

Explore some of the options for the compositor::Builder.

Reimplement build_auto using the explicit build functions.

Use $DISPLAY to detect when running nested in X11 and $WAYLAND_DISPLAY to detect running nested in Wayland.