2020 M1 MacBook Pro: Built-in display does not show entire desktop (truncates 300px from right and 200 px from bottom)
Previous reboots did not recover desired behaviour. But having checked the presence or absence of symptoms during Recovery Mode as per Andreas Ley's comment in my original question (behaviour was normal), I reboot again into normal mode and correct behaviour remained.
So I don't know specifically what was at fault, but an interim Recovery Mode reboot helped to remedy the situation.