Black edge around screen

A couple of months ago an update to my iphone 11 reduced my screen size! I now have a fat black strip all the way around, I've lost near a cm of screen use, the active screen used to go pretty much all the way to the edge of the glass. I did several searches of the internet, feeling certain I'd find a solution, but then other things kept me busy and I didn't get around to returning here to report this.

I'm now on iOS 17.5.1. but the update that caused this change was months ago.

Other somewhat related posts have suggested checking "accessibility" and "zoom" features, but I use neither, and I've double checked that none of that is happening.

My phone is always updated instantly, and gets rebooted at least once a week.