Yes, `doc.parentWindow.attachEvent( "onbeforeunload", setDocument )`, which will set `document` to `preferredDoc` and update dependent variables upon `onbeforeunload`, just as happens upon [initialization](https://github.com/orkel/sizzle/blob/8207a90f4dad704d2591d1fb03e4d05dc7fd1e23/sizzle.js#L1988). Where do you think that will throw?