UserAgent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/53.0.2785.101 Safari/537.36

Steps to reproduce the problem:
1. https://jsfiddle.net/crl/0w9oge6g/19/
2. 
3. 

What is the expected behavior?
there should be no gap between the red button and the box-shadow of its parent

adding a backgound on the parent div.buttwrap make this gap less visible https://jsfiddle.net/crl/0w9oge6g/20/, but if you look closely there's still one.

Not tested on Canary, but I think it's the same

What went wrong?
There a small white gap between them: http://i.stack.imgur.com/naXHU.png

I've tried background-clip too, or forcing a border in red, but that doesn't change.

I wonder if it's a bug though

WebStore page: https://jsfiddle.net/crl/0w9oge6g/19/

Did this work before? N/A 

Chrome version: 53.0.2785.101  Channel: stable
OS Version: 10.0
Flash Version: Shockwave Flash 22.0 r0

not tested on Firefox