Perfect!  Reminds me of Master Pages for older .NET applications.  This is what I suspected but since I didn't know the term "pane" I couldn't search the documentation.  Thank you!

Perhaps the docs I linked can be updated to reference the page on "panes"

This seems to happen on every form.  Also, once you backspace all the content you cannot get the curser back in the editor without refreshing the page.

So is the <form> required or can you specify the data controller some other way?

Looks like they removed this from all but the main index.  But you're right, the quality of the photo is terrible.

Related: There is no feedback when you rate a post.  I had to refresh the page before I was sure it even worked.