UserAgent: Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/60.0.3112.78 Safari/537.36

Steps to reproduce the problem:
1. Register an event listener for keydown and keyup for the shift key
2. Press shift
3. Keyup event will never be executed

var shiftDown = false;
var setShiftDown = function(event){
    if(event.keyCode === 16 || event.charCode === 16){
        window.shiftDown = true;
    }
};

var setShiftUp = function(event){
    if(event.keyCode === 16 || event.charCode === 16){
        window.shiftDown = false;
    }
};

window.addEventListener? document.addEventListener('keydown', setShiftDown) : document.attachEvent('keydown', setShiftDown);
window.addEventListener? document.addEventListener('keyup', setShiftUp) : document.attachEvent('keyup', setShiftUp);

setInterval(() => console.log(shiftDown), 1000);

What is the expected behavior?

What went wrong?
This occurs in Chrome as well as in Chromium. Might be related to another library? And it doesn't occur on macOS. 

Did this work before? Yes 

Chrome version: 60.0.3112.90  Channel: stable
OS Version: Arch Linux
Flash Version: