Pressing 2 keys, release 1, produces keydown of other key again

问题

I press S, followed by D, on my keyboard, and release S. Why does the following code trigger a key down event for D once again, after I release S? Is there some way to find out that the second down event for D was caused by a key up of another key?

function handler(event) {
  if (!event.repeat) console.log(event.code, event.type);
}

window.addEventListener('keydown', handler);
window.addEventListener('keyup', handler);

Output:

KeyS keydown - S pressed
KeyD keydown - D pressed
KeyS keyup   - S released
KeyD keydown - WHY???

The strange keydown event only happens in Chrome (I'm using Chromium 66.0.3359.139 on Linux Mint), this does not happen in Firefox.

Might this actually be a bug, or is it intended behavior?

来源：https://stackoverflow.com/questions/50372849/pressing-2-keys-release-1-produces-keydown-of-other-key-again