Visual diff is not working on mkdocs, double-selects root element

[reteps]

Details

• Read the Docs project URL: https://prairielearn--11830.org.readthedocs.build/en/11830/question/index.html?readthedocs-diff=true
• Config: https://github.com/PrairieLearn/PrairieLearn/blob/master/.readthedocs.yml

Expected Result

When I click visual diff, it diffs correctly. Instead, it currently throws an error.

Actual Result

this.performDiff(this.cachedRemoteResponse.content);
// ...

    const htmlDocument = parser.parseFromString(remoteContent, "text/html");
    const oldBody = htmlDocument.documentElement.querySelector(
      this.rootSelector,
    );

The JS fetches this url: https://prairielearn--11830.org.readthedocs.build/_/api/v3/embed/?url=https%3A%2F%2Fprairielearn.readthedocs.io%2Fen%2Flatest%2Fquestion%2Findex.html&maincontent=main+%3E+div+%3E+div.md-content

The content it fetches is at the root selector:

It then tries to select the root node of the fetched document, and fails, since it has already fetched it!

Fix: don't select content at the root selector once it has been fetched through the embed link.

[reteps]

This looks like a regression in a6623bf.