R1:e9c7310e836f

Fix Conpherence scroll behavior in Firefox/Chrome

Summary: Ref T7825.

Test Plan:
- In Firefox, Chrome, and Safari:
- With a mouse and no mouse:
- Sent messages in Conpherence.
- Scrolled up and down in a thread.

Reviewers: chad, btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T7825

Differential Revision: https://secure.phabricator.com/D12416