Nav scrollbars only shown when needed

Saw the demo via Anchor Themes. It annoyed me that you could see the disabled scrollbars on the nav area on the left; i.e. overflow: scroll, so I fixed it. So now it doesn't show the scrollbars, unless say you reduce the height of the viewport to less than the height of the nav menu, then ( _only_ ) the vertical scrollbar is shown. I only tested this via Chrome's devtools but should be fine. Only a small change that all browsers support.