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On Linux, you can try Xephyr (https://www.freedesktop.org/wiki/Software/Xephyr/, https://wiki.archlinux.org/title/Xephyr). It's not as nice as DeskPad, but you can basically achieve the same thing.

Xephyr runs a whole new X server - it's not as easy to drag and drop an application into the nested server, it has to be launched with DISPLAY=:1.