That can't be full screen if I can see another program behind it? or is that your taskbar on the left?
Sure its not chrome? I notice it adds a line to its edge if not in focus. Is it on all pages?
some sites already have borders too
not as obvious as yours though.
have you tried Chromium Edge? Its essentially Chrome with a few differences, one being I don't see a line on side if I unfocus it.