R1:9ba430cca9e3

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.