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