Per changes in https://github.com/whatwg/dom/pull/353 we should fire slotchange event that bubbles exactly once.
Created attachment 294902 [details] Fixes the bug
<rdar://problem/29281063>
Comment on attachment 294902 [details] Fixes the bug Clearing flags on attachment: 294902 Committed r208817: <http://trac.webkit.org/changeset/208817>
All reviewed patches have been landed. Closing bug.