Fix mouse access to Workboard scrollbars

Summary: Fixes T7075. The invisible "fancy" scrollbar was covering these; hide it more aggressively.

Test Plan:

• Scrollbars on Workboards can now be interacted with directly.
• Normal scrollable and unscrollable pages work as expected.
• Resized some windows.

Reviewers: btrahan, chad

Reviewed By: chad

Subscribers: epriestley

Maniphest Tasks: T7075

Differential Revision: https://secure.phabricator.com/D11560