How do I get rid of the black bar on the right side of my browser?

Not only did this mysterious black vertical bar suddenly appear on the right side of the browser window one day, my cursor is offset: to click on something on a web page, I have to position the cursor arrow a little to the right of what I want to click on. I had this problem before and was somehow able to fix it but I forgot what I did so I am hoping to find the answer here as it is very annoying. There is not problem with the internet header at the top of the page - just the internet window where the web pages are.