On a few occasions I get two black bars at the right side of the tab that scroll with the page, but disappear if I change or resize the tab or open a page other than XenForo. It also happens here at XenForo.com
Google Chrome Version 61.0.3163.100 (Official Build) (64-bit) on Windows 10.0.15063 Build 15063. It might be a rendering issue, as I haven't been able to track it down in the DOM-inspector, but I've only ever encountered it on XF2 with at least Beta 3, either here or on my local developer installation, and nowhere else. It is rare though, and probably goes away unnoticed on many occasions. Haven't found out how to trigger it though, as of yet. It mostly just happens by scrolling the page, but I'll try to recall what I did the next time I'll encounter it.