It is possible that something broke by changing a number of screens, although that would be odd, since this is a separate scroll bar from the others being used. If you can reproduce it again, that would be great.
I don't really know on what point it came there, I just noticed it. I tried opening various things, switching tabs, scrolling out/in. Can't reproduce it. I'll keep my eye on it but I feel like it won't appear ever again..