the wubi installer. i remember that. installed ubuntu inside a container file and added boot entry so could choose at boot time. yea, that was never really supported when it was offered but it mostly worked.

endless os does have that option yet today. and it does work–with some caveats: it’s slow af with a hdd. well, immutable distros in general are, but this install method just makes it worse. and your windows c: cannot be encrypted (e.g. bitlocker). also be aware that the next version (endless 7) that’s in development will change pretty much everything ‘under the hood’ that makes endless what it is, as they’re moving from their customized gnome and in-house immutable debian base to an immutable based on gnome os itself.