The post on SO looks like a Tumbleweed badge candidate. Not much luck elsewhere either, I see. Strange that the reply on Daniweb mentions not seeing the issue.
I’ve generally experienced border, transform and overflow having weird interaction. Although I’m not aware of the exact specification causing that. But the fact that changing the background also has an effect, it’s most flabbergasting to me.
Hope you’ll figure it out (please report back if you do).