Xnest(1) on Linux
The text 087a38fac3a1, as Debian 11 (Bullseye) shipped it.
a nested X server
Flags it lists
-
-bw -
-class -
-depth -
-display -
-full -
-geometry -
-install -
-name -
-parent -
-scrns -
-sss -
-sync