The article on a page with a lot of content reflows after load

If a page has a lot of content, the browser will start the render the visible content right away. When it then discovers there's a right sidebar (the toc), it will reflow the article to make room for it. This can be fixed by putting the toc insertion slow before the article, then using flexbox to swap the order. That way, the browser preserves space for the toc and avoids the reflow.