MutationObserver to detect change in null child element

I have an iframe that is rendering content from another domain. Obviously I cannot access the contentDocument from the other domain (returns null). But when a user performs an action on the rendered iframe, it redirects back to my domain. So at that point I am able to read the document. Basically once the contentDocument is not null, I want to perform an action.

Currently I'm just trying to fire an alert if a child of the iframe (in this case its contentDocument mutates...

window.onload = () => {
  setTimeout(initializeObserver, 2000)
}

const initializeObserver = () => {
  const iframe = document.getElementById('my_iframe')

  observer.observe(
    iframe,
    {
      attributes: true,
      childList: true,
      subtree: true
    }
  )
}

const observer = new MutationObserver(mutations => {
  mutations.forEach(mutation => {
    if(mutation.addedNodes.length) {
      alert("SOMETHING HAPPENED!")
    }
  })
})

But the alert never fires even when child nodes are added and elements within the iframe change. I've read a lot of examples, but maybe I'm still missing how this works. Can someone help explain why this observer isn't triggering on the mutation?

Thanks in advance

When a browsing context in an <iframe> has its location changed, the src attribute of the containing <iframe> doesn't change, so your page can not see any difference.

What you could do since you apparently control the other page is to post a message to your main page.
Then you just have to listen for that message and you know your frame has come back.

// in main
addEventListener("message", (evt) => {
  if( evt.origin === location.origin && evt.data === "I'm back" ) {
    // your page came back
  }
});

// in framed page
top.postMessage( "I'm back", location.origin );