A small gray, vertical strip appeared close to the left side of the screen on a macbook. How to remove?

Question

macbook

A small gray, vertical strip appeared close to the left side of the screen on a macbook. Most likely this happened after a one-time connection of the monitor. It is visible in all full-screen applications but slightly changes its position and length. How to remove and what is it? (She's not usable)

Answer 1

15432 , helped solve the problem by dropping the article:
https://apple.stackexchange.com/questions/113788/g...
In which there was this answer:
You could always disable the multi-menubar: system pref > mission control > and uncheck "displays have separate spaces"
– y3sh Dec 29 '13 at 16:45 use shared space" (Something like that)