Fix Conpherence durable scrollbar with no USB devices

Summary:
Ref T7014.

When no devices are connected, we don't meaningfully activate JX.Scrollbar (by design), but there was no fallback overflow CSS.

Also massage a couple of other CSS things to get sliiightly less broken behavior.

Test Plan:

• Disconnected USB devices; scrolled.
• Connected USB devices; scrolled.

Reviewers: btrahan, chad

Reviewed By: chad

Subscribers: epriestley

Maniphest Tasks: T7014

Differential Revision: https://secure.phabricator.com/D12008