There is abug in Chrome where inheritance of box-sizing does not happen correctly with thedetails element. This results in allinput (and every other element that iswidth: 100%) overflowing thedetails.

This is demonstrated bythis codepen.

The solution is to add:

details > * {  box-sizing: border-box;}

Would post a PR, but can't get grunt working locally and don't have time to fix it now.