[LinuxBoy-96]

• Problem: the compositor tells systemd once that it's ready (NotifyState::Ready), with no WATCHDOG=1 after that. cosmic-session reacts only when it exits.
• Consequence: any compositor freeze leaves the desktop frozen until someone kills the process by hand, or reboots. Switching to a console goes through the frozen compositor and the power button is blocked too, so in practice that means SSH from another machine, or a hard power-off.
• Fix: a watchdog thread like the panel's (abort if the loop hasn't turned for N seconds), or systemd's watchdog.

Related: pop-os/cosmic-session#240 (the session comes back even if the new cosmic-comp can't start), pop-os/cosmic-session#241 (the power button works while cosmic-comp is frozen).

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