I can't tell if this is purely an Opera bug, but it just started showing up today shortly after I was seeing some "What's new in Chrome 60" announcements in Inspector, so I'm assuming it might be from that.
When in the admin of my Xenforo forum, I'm seeing this weird looking scrollbar pop up in some pages, other pages it looks normal:
I've checked in other browsers, the same page and menu, and it doesn't do that. It just started happening today. Any ideas?