Inadvertent iMac zoom

Every now and then, but twice today so far, as I am scrolling on Safari the display suddenly zooms in so that the former edges of the display disappear. So far the only thing that gets the display back to normal is to restart the iMac.

This is a 27-inch 2020 iMac and I use a Magic Mouse. I suspect that it's something I'm doing with the MM that's the problem but I have no idea what.