Fix scrollbar in draggable area.

Summary: This starts the content over 7 px instead of overlapping the drag bar on the side menu.

Test Plan: Tested it in Chrome with scrollbar

Reviewers: epriestley, btrahan

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T2322

Differential Revision: https://secure.phabricator.com/D4449