Festering iMac has failed to start up properly, stalled partway through the setup after logon. I do recall it took its own sweet time in shutting down last time on Friday.
Rebooted, even worse. Just a desktop picture, nothing else.
Rebooted from a cloned external SSD & used Disk Utility to run First Aid then hecked the directory structure with TechTool Pro. No errors in directory.
Rebooted again, thus time at least it has worked.