Transparent rectangle in the left hand top corner of my Mac.

Please tell me what this rectangle in the left hand corner of my mac is an how to get rid of it. It's been there for months. I have scoured the internet for an answer to no avail and none of the responses to similar questions in this forum have been any help, despite trying them all.

It started when I was on Mojave and stuck around after upgrading to Catalina (although I wish I'd never done that... I could go on and on about how much trouble I've had since upgrading, but will save it for another time.)

Please tell me someone out there has an answer that can correct this issue.

see photos below.

This one is from my phone and shows the left rectangle.

This one is a screen shot and nothing appears here, even though it's there.