Created by: cvrebert
On the grounds that it's what we're advising former Affix users to migrate to in v4, and it would be great if polyfills for it stopped being necessary.
Refs https://wpdev.uservoice.com/forums/257854-microsoft-edge-developer/suggestions/6263621-position-sticky Refs https://crbug.com/231752
CC: @twbs/team for review