Table of Contents Knowledge base

Not as a widget, no. A contents list in a sidebar has to be built from a post that has not been rendered yet, which is how those end up out of step with the article.

Settings → Bring it back once it has scrolled off does the job that a sidebar list is usually wanted for: a button in the corner reopens the contents from anywhere in the article, without a sidebar and without moving the list out of the post.