UserAgent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Firefox/60.0

Steps to reproduce the problem:
1. Create a custom element with an attached shadow root 
2. Attach an event listener to listen to the selectionchange event from the shadow root
3. Add that element to the DOM.

Simple code to reproduce:

customElements.define("test-element", class extends HTMLElement {
  constructor () {
    super()
    this.attachShadow({mode:"open"})
  }

  connectedCallback () {
    this.shadowRoot.innerHTML = "<div contenteditable='true'></div>"
    this.shadowRoot.addEventListener("selectionchange", () => {
      console.warn("Selection modified")
    })
  }
})

document.body.innerHTML += "<test-element></test-element>"

What is the expected behavior?
The selectionchange event should fire when a [contenteditable] element is modified inside of a shadow root.

What went wrong?
The event didn't fire fire. It does however, fire on the global Document.

Did this work before? No 

Does this work in other browsers? N/A

Chrome version: 64.0.3282.186 (Developer Build) (64-bit)  Channel: n/a
OS Version: 4.15.6-1-ARCH
Flash Version: