systemd-xdg-autostart-generator-ed service does not correctly run script that detaches a subprocess

[Yann Salmon]

I have a script that basically does

```
#! /usr/bin/bash
sleep 5
thunderbird &
```

and an appropriate desktop file in .~/.config/autostart to run it.

Since upgrading from Plasma 5.24 to 5.27 (from the backports PPA), this no longer works. The script is run (added logging, eg. with systemd-cat, prove it) but Thunderbird does not appear. However, it does work if I change it to the following.

```
#! /usr/bin/bash
sleep 5
thunderbird
```

So I suspect this is related to how systemd-xdg-autostart-generator runs the associated service. I tried various combinations like ``kstart5 thunderbird & disown``, to no avail.

Note this script is an MWE example ; my actual script is more convoluted and needs subprocess detachment to work properly.

Plasma 5.24 did not exhibit this behaviour. I do not know if it was using systemd-xdg-autostart-generator to manage autostart.

Current systemd version is 249.11-0ubuntu3.12, which is too old to report upstream about.

Operating System: Kubuntu 22.04
KDE Plasma Version: 5.27.10
KDE Frameworks Version: 5.104.0
Qt Version: 5.15.3
Kernel Version: 5.15.0-100-generic (64-bit)
Graphics Platform: X11

[Ubuntu Foundations Team Bug Bot (crichton)]

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