callback - wait for Element Upgrade in connectedCallback: FireFox and Chromium differences

Question

Update March 2021:

FireFox bug fixed, now behaves the same as Chromium and Safari.

That means waiting for the JS EventLoop to be empty (with setTimeout or requestAnimationFrame) in the connectedCallback is now a cross-browser method

connectedCallback(){
 setTimeout(()=>{
   // can access lightDOM here
 }); // ,0 not required
}

What the heck is the Event Loop? - Philip Roberts
https://www.youtube.com/watch?v=8aGhZQkoFbQ

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Update Oct. 28 2020:

• Now reported by Mozilla engineer Anne van Kesteren as a bug in FireFox:

  FireFox invokes the connectedCallback too late:
  https://bugzilla.mozilla.org/show_bug.cgi?id=1673811

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First post May. 2020:

Bitten again by this Chrome Element upgrade issue, after spending a week in FireFox.

Forgot to wrap code in a setTimeout before delivering to Chromium browsers.

• FireFox prints: ABCD

• Chromium prints: ADCD

Question: Why the difference?

<script>
  customElements.define('my-element', class extends HTMLElement {
    connectedCallback() {
      console.log(this.innerHTML);// "A" in FireFox, "" in other Browsers
      if (this.innerHTML == "A")
        this.innerHTML = this.innerHTML + "B";
      else
        setTimeout(() => this.innerHTML = this.innerHTML + "D");
    }
  })
</script>

<my-element>A</my-element><my-element>C</my-element>

Answer 1

I think the Chrome/Safari behaviour is less intuitive for the beginners, but with some more complex scenarios (for example with child custom elements) then it is much more consistant.

See the different examples below. They act strangely in Firefox...

Another use case that I don't have the courage to code: when a document is parsed, maybe you don't have the end of the document yet. Therefore, when a custom element is created, you cannot be sure you get all its children until you get the closing tag (that could never arrive).

According to Ryosuke Niwa for WebKit:

The problem then is that the element won't get connectedCallback until all children are parsed. For example, if the entire document was a single custom element, that custom element would never receive connectedCallback until the entire document is fetched & parsed even though the element is really in the document. That would be bad.

So it's better no to wait and connect the custom element as soon as it is created, that means with no child.

<script>
    customElements.define( 'c-e', class extends HTMLElement {} ) 
    customElements.define('my-element', class extends HTMLElement {
      connectedCallback() {
        console.log(this.innerHTML, this.childNodes.length)
        let span = document.createElement( 'span' )
        if (this.innerHTML.indexOf( 'A' ) >= 0 )
            span.textContent = 'B'
        else
            span.textContent = 'D'
        setTimeout( () => this.appendChild( span ) )
      }
    })
</script>
<my-element>A</my-element><my-element>C</my-element>
<br>
<my-element><c-e></c-e>A</my-element><my-element>A<c-e></c-e></my-element>
<br>
<my-element><c-e2></c-e2>A</my-element><my-element>A<c-e2></c-e2></my-element>