black rectangle outline on the screen

After I updated to Mac OS 10.13.3 I was getting a black rectangle outline on the screen. I spoke to 2 Apple advisers who had never heard of it. I finally found the solution in System Preferences > Accessibility > Zoom > Options, there was a selection for "show preview rectangle when zoomed out". I unchecked that and the problem disappeared. This is not a question, it's to help anyone else who happens to have the same problem.