I'm not exactly sure since when, but for a while I had had a new behavior: logging out from a session that has some 
`screen` or `tmux` sessions launched will terminate those too, which beats one of the purposes of running stuff in them.

There is a workaround, which you can find everywhere:

    loginctl enable-linger $USER 
    systemd-run --scope --user screen
    
This is good for launching new sessions, but what if you already have a session running, you're out of luck.

Or are you?

Unluckily `systemd` does not have a nice way to do this. But it is still possible.

We can create a new scope and attach the `screen` master later:

    systemd-run --scope --user sleep infinity
    # Running as unit: run-p1787326-i1787327.scope; invocation ID: ed6adac8a0db4e8f9e9ce59731cc2aa5
    
We use `infinity` so the scope does not finish before we can attach to it. Now let's find the control group under which 
it's running:

    systemctl --user show -P ControlGroup run-p1787326-i1787327.scope
    # /user.slice/user-1000.slice/user@1000.service/app.slice/run-p1787326-i1787327.scope

This references a scope directory in the `/sys/fs/cgroup/` directory. We need to add the PID of the `screen` to the 
`cgroup.procs` file in it:

    echo 1789758 > /sys/fs/cgroup/user.slice/user-1000.slice/user@1000.service/app.slice/run-p1787326-i1787327.scope/cgroup.procs

Thanks to `grawity#systemd@libera.chat`.