Per https://dom.spec.whatwg.org/#dom-event-cancelbubble setting cancelBubble to false should do nothing. WPT: https://github.com/w3c/web-platform-tests/pull/4304.
Created attachment 297956 [details] Patch
Comment on attachment 297956 [details] Patch Clearing flags on attachment: 297956 Committed r210254: <http://trac.webkit.org/changeset/210254>
All reviewed patches have been landed. Closing bug.