[FEATURE] What should happen when ToC is long enough to have a scrollbar

[jesse-wei]

On posts/setup_site, there's a scrollbar on the ToC since there are so many headers. When you scroll past the last element, the ToC doesn't automatically scroll down.

It'd be distracting to automatically scroll the scrollbar. A solution would be to decrease the depth of the ToC, but this doesn't feel optimal.

[jesse-wei]

Decreasing depth of the scrollbar does not work. It completely breaks the active element highlighting as soon as you reach a heading that's not shown in the ToC.