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.