See shadow-dom/event-composed-path.html in wpt. See https://github.com/whatwg/dom/pull/699 for the new DOM standard prose around this. Currently Firefox leaks closed trees it seems.
Pushed by email@example.com: https://hg.mozilla.org/integration/mozilla-inbound/rev/e4f3ad7a95af bug 1516289 - Fix composedPath implementation when slots are present. r=smaug
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