Here's a comment about this one weird trick we use to know about nodes in the walker

I guess the comment makes sense, but I'd feel better if @campd would look at this.

I think the comment is required to help understand how this all works. I was a bit surprised at first to see that DOM.querySelectorAll would only return nodeIds, not handles, and even more surprised to see that our refmap would already contain handles for all these ids, even though these were deep nodes not displayed yet in the markup-view.