Ah,  bug 884846  happens just for custom scrollbars? I'm not sure how much work it would be to make custom scrollbars use scrollbar layers - I suspect it may be quite involved. If you'd like to investigate and see I think it'd be nice to prevent future surprises like this.

But to fix  bug 884846  I think it might just be easier to have IsDrawnScrollbar know about the difference in custom scrollbars here (by checking for the CustomScrollbar main thread scrolling region, for example).

Yes, I just add this bug as a todo. not blocking 884846.