I had the same Mac on HIgh Sierra and had the same problem using a Sony tv as a monitor. From what I could find out, it¡¯s the TV that¡¯s the problem. ?There¡¯s some disagreement between Sony and Apply on just what HDMI is, and maybe the same applies to vizio. ?There¡¯s some function in TVs to fill the screen, even when that means either the top and bottom or the sides are cut off. ?Maybe the vizio has a setting that defeats that ¡ª the sony doesn¡¯t ?I found there are some resolutions the two can agree on, and using those solved the problem. ?I have updated this Mac to 10.14 so it can use a particular app, but that didn¡¯t affect this problem. ?
On updating the OS for the sake of updating the OS, I¡¯d suggest you don¡¯t bother unless there is some app you want to use that needs it. ?I had this Mac on Catalina for a bit, and more problems arose than were solved, generally in networking. ?. ?
