Fix firefox workboard half-pixel issue

Summary: Fixes T8827. Firefox and other browsers differ here. Going to make the header relative so boards always slide under it.

Test Plan: Test workboard with scrolling in FF and Chrome.

Reviewers: epriestley, btrahan

Reviewed By: btrahan

Subscribers: epriestley, Korvin

Maniphest Tasks: T8827

Differential Revision: https://secure.phabricator.com/D13630