Prevent text selection (mostly) on workpanels

Summary: Fixes T5719, Not completely convinced we won't see another ticket here, but overall testing it feels better.

Test Plan: dragon drop a lot of stuff.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: epriestley, Korvin

Maniphest Tasks: T5719

Differential Revision: https://secure.phabricator.com/D10062