Screen sharing - stuck in full size

We have a MacBook Air running High Sierra 10.13.6 on which I've been successfully, for months, sharing the screen of another Mac, that is using an external 1920x1200 monitor, to the MacBook Air. In order to be able to see the entire desktop of the Mac with the big monitor in the window on the MacBook Air, I choose "Scale to fit available space".

Last night, that preference suddenly stopped working on the laptop, even though I did not turn off scale to fit available space. It just happened spontaneously. In other words, to see the screen of the other Mac, I had to scroll side to side and up and down. And toggling back and forth between Show full size and Scale to fit available space made absolutely no difference. It is permanently set on Full size.

First of all, is this more likely some sort of an issue with the computer that is sharing its screen, or the laptop that is viewing the screen?

I have not (yet) tried some of the usual troubleshooting approaches - trying it in Safe Boot, or seeing what happens with a new temporary user.

If I were to find out the problem lies with original user's profile, does anybody know the name of the preferences file for Mac OS screen sharing.

Thanks for looking,