Conpherence - fix durable column bug where scrolling broke over embedded mock

Summary: Fixes T7842.

Test Plan: using chrome, successfully scrolled durable column despte encountering embedded mocks

Reviewers: epriestley, chad

Reviewed By: chad

Subscribers: Korvin, epriestley

Maniphest Tasks: T7842

Differential Revision: https://secure.phabricator.com/D12612