As of 2020-03-23 I intend to turn "Document as explicit root of an
intersection observer" on by default on all platforms. It has been
developed behind the
dom.IntersectionObserverExplicitDocumentRoot.enabled preference. Status
in other browsers is: landed in WebKit development build without flag ;
shipped in Chromium since version 81.
Bug to turn on by default:
This feature was previously discussed in this "Intent to prototype"