UserAgent: Mozilla/5.0 (Windows NT 10.0; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/48.0.2564.116 Safari/537.36

Example URL:
http://codepen.io/anon/pen/oxzvbQ

Steps to reproduce the problem:
1. visit http://codepen.io/anon/pen/oxzvbQ
2. observe there is a scrollbar which covers partially the word "close"
3. try to comment any of the three lines:
a) 14. -webkit-backface-visibility: hidden;
b) 17. z-index:0;
c) 18. position:relative;
and observe that this causes the scrollbar to move to the background (underneath the word "close").

What is the expected behavior?
Latest Firefox and the latest IE do not cover the word "close" with a scrollbar. The .popup covers the scrollbar in them. I would expect the same with Chrome.

I would expect the behaviour to be the same regardless of using -webkit-backface-visibility:hidden.

I would expect that a scrollbar which is "belongs" to a different element (in this case probably it is a scrollbar of .app) not to be displayed on top of a fixed element. 

I would expect that a scrollbar which for some reason appears to visible and enabled, should actually work when I use mousewheel or drag it with mouse.

What went wrong?
In Chrome the scrollbar appears on top of everything and appears like it is enabled (based on it's colors) but actually I can not scroll it (neither using mousewheel nor by dragining), and when trying to inspect this area using dev tools it seems that this part of screen is being considered to belong to ".popup" or ".close".
Moreover I can cause the scrollbar to appear below the surface of .popup by simply removing -webkit-backface-visibility:hidden directive which seems to support a hypothesis that this is some rendering artifact, and not a real UI control.

Does it occur on multiple sites: Yes

Is it a problem with a plugin? N/A 

Did this work before? Yes I think it used to work well a month ago (but can not prove it)

Does this work in other browsers? Yes 

Chrome version: 48.0.2564.116  Channel: n/a
OS Version: 10.0
Flash Version: Shockwave Flash 21.0 r0