The same issue as Why was a vertical line added on the left for users who collapse the left navigation? Can it please be removed? has arisen again.
The bug was initially fixed, but it seems to have regressed. I'm posting again because SE generally prefers we file new bug reports if a fixed bug comes up again, rather than removing the status-completed tag on the older one.
Again, I really don't like it there. It's eye-catching and extremely distracting for me.