There are a few places where we check document attachment with jQuery.contains( elem.ownerDocument, elem ) (isHiddenWithinTree for show/hide, curCSS for inline style fallback, buildFragment/domManip/remove/jQuery.clone for managing script evaluation during DOM manipulation), all of which fail inside Shadow DOM:

$.contains(document, elem) will return false when elem is shown in a shadow dom.
change to elem.getRootNode({composed: true}) === elem.ownerDocument works.

We should investigate the cost of a remedy (e.g., a Shadow-DOM-compatible isAttached( elem ) function).

Yes to going forward and investigating, but we may scrap it later depending on how involved this gets.

As noted in the meeting, we'd probably benefit from an internal isAttached even if it doesn't handle shadow DOM, because it better communicates intent.