Spec: DOM Standard: https://dom.spec.whatwg.org/#dispatching-events WPT Update: https://github.com/w3c/web-platform-tests/pull/3865 So far, Blink does not dispatch synthetic events if all of the following conditions are satisfied: 1) event's target is in a shadow tree 2) event has relatedTarget 3) event's target and event's relatedTarget are identical The new behavior will dispatch synthetic events even if the all conditions are satisfied.