Fix scrolling over sidenav

Summary: This is the simplest fix I could find.

Test Plan: Scroll over sidenav on home, maniphest, workboards, etc. Test mobile, deskop. Firefox, Safari, Chrome, Internet Explorer.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D11881