UserAgent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:58.0) Gecko/20100101 Firefox/58.0

Steps to reproduce the problem:
I made a codepen where you can see the problem when hovering or clicking on the select:
https://codepen.io/seveves/pen/MORjNB

What is the expected behavior?
Don't change anything on the page.

What went wrong?
The mouse interaction with the select input has impact on another containers size.

Did this work before? N/A 

Does this work in other browsers? Yes

Chrome version: <Copy from: 'about:version'>  Channel: stable
OS Version: 10.0
Flash Version: