"On setting, if the new value is null then the c..."

[cdumez]

https://html.spec.whatwg.org/#navigating-auxiliary-browsing-contexts-in-the-dom:disowned-its-opener

On setting, if the new value is null then the current browsing context must disown its opener; if the new value is anything else then the user agent must call the [[DefineOwnProperty]] internal method of the Window object, passing the property name "opener" as the property key, and the Property Descriptor { [[Value]]:​ value, [[Writable]]:​ true, [[Enumerable]]:​ true, [[Configurable]]:​ true } as the property descriptor, where value is the new value.

Consider the case:
// window.opener returns a [Window]
window.opener = "foo"
// window.opener returns "foo"
window.opener = null;
// This disowns the opener as per the specification
// Should window.opener return null or "foo"?

My interpretation of the specification is that we should return "foo". This is because we disown the opener but do not call [DefineOwnProperty] because [DefineOwnProperty] is only called if the value is anything else than null.

However, quick testing in Firefox and Chrome seems to indicate they return null.

[cdumez]

cc @domenic

[cdumez]

Actually, upon further testing, I may be mistaken. When setting the value to "foo" in the first place, it calls [DefineOwnProperty]. So when you set it to null the second time, it appears to updates the descriptor's value to null but does not actually disown the opener.