Chrome Version: 58.0.2991.0
OS: macOS 10.12.2

What steps will reproduce the problem?
(1) Open http://jsbin.com/vekiji/edit?html,js,console,output in Chrome
(2) Observe the console output

What is the expected result?
"AFTER: type=foo"
"AFTER: bubbles=false"
"AFTER: cancelable=false"
"AFTER: detail=origDetail"

What happens instead?
"AFTER: type=foo"
"AFTER: bubbles=false"
"AFTER: cancelable=false"
"AFTER: detail=newDetail"

In other words, the last line should instead be "AFTER: detail=origDetail".

Further details:
This is a simplified version of the test in
http://w3c-test.org/dom/events/Event-init-while-dispatching.html
that fails with:
Fail	Calling initCustomEvent while dispatching.
	assert_equals: initCustomEvent detail setter should short-circuit expected (object) null but got (number) 1

Both tests call initCustomEvent() on an already-initialized CustomEvent while it's being dispatched.
Per step 1 of https://dom.spec.whatwg.org/#dom-customevent-initcustomevent ,
calling .initCustomEvent() on an event while that event is being dispatched should have no effect.
(See also step 1 of https://dom.spec.whatwg.org/#dispatching-events .)
Instead, Chrome is setting the CustomEvent.detail property to the value given in the initCustomEvent() call.