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