R1:33a3bbbd7a3a

Put scrollbar underneath haunted comment panel

Summary: Fixes T7513. This is consistent with the behavior of an OS scrollbar.

Test Plan: Scrolled with haunting.

Reviewers: btrahan, chad

Reviewed By: chad

Subscribers: epriestley

Maniphest Tasks: T7513

Differential Revision: https://secure.phabricator.com/D12020