Created attachment 118590 [details] [review]
When starting from the .desktop file, use D-Bus activation

This ensures that syncevolution is consistently a child of systemd
(if using the systemd user service) or dbus-daemon (otherwise).

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I'm less sure about this one. It makes sure only one instance gets started, but
it does rely on having dbus-send(1) installed.

Another possibility would be something like

if [ -d "$XDG_RUNTIME_DIR" ] && [ -d "$XDG_RUNTIME_DIR/systemd" ]; then
    exec systemctl start syncevo-dbus-server.service
elif command -v dbus-send >/dev/null; then
    exec dbus-send [...]
else
    exec @libexecdir@/syncevo-dbus-server [...]
fi

but maybe that's too much complexity?