Is null document.location expected in this case or a bug?

In appcontent.DOMContentLoaded for an iframe, which in an inline script 
changes parent.location and then assigns its own location to about:blank, 
document.location is null, window.location is about:blank and document.URL 
is the actual url.

Even though location has been changed before DOMContentLoaded, the 
navigation presumably has not started, so should location be something other 
than the actual url of the document?

Why are document.location and window.location out of sync?

Why is document.location null?