As of now, the footer is separated from the document layout. When you scroll to the bottom of the page, the top of the table of contents becomes hidden. The table of contents on the left side should always remain fixed. Right now, when you scroll down to the bottom of the page, it looks as if the entire layout of the page shifts up slightly and it makes the layout appear broken.
Instead of
<div>
<aside />
<main>
...content
</main>
</div>
<footer />
It should be
<div>
<aside />
<main>
...content
<footer />
</main>
</div>
That way scrolling to the bottom of the page won't jolt the layout around a bunch.