It seems like I have a desktop which is slightly larger than my screen. When I maximise a window the bottom portion (approx 300 pixels) and right portion (approx 200 px) are not visible. I can move the mouse to that invisible and perform click operations but the pointer is not visible on screen. (It is shown when in the visible region). Attempting to scroll beyond the visible extremities does not pan the screen.
See the attached screen shots. For the truncated ones, I captured a portion of the visible regions, starting at the top of the screen and moving to the right-most visible (or bottom most). For the non-truncated ones I moved the mouse enough such that I could be confident of being at the extremity of the desktop – but I had to do this unseen.
The same thing happens with vertical truncation, but the images take up a lot of vertical space in this post and don't really add much (though I can add them if this is desired).
I've had to move the Dock to the left of the screen in order to make it visible but I would prefer to keep it on the bottom.
In terms of my machine setup:
I'm using a new 2020 issued M1 MacBook Pro running Big Sur. I've tried both with and without an external display. With an external display, that external display shows fine (and has no truncated region). But in both setups, the built-in display of the laptop is truncated.
In case it matters: I migrated my data to it from a 2015 edition MacBook Pro which had been upgraded to Mojave.
I've tried looking in System Preferences but don't see anything related to screen size in the Displays subsection. I've also tried looking for zoom/magnifying settings (such as might be used for a11y reasons), again to no avail.
I'd welcome suggestions for things to try here.
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.