This was noticed by Bob Owen in https://bugzilla.mozilla.org/show_bug.cgi?id=1190641#c20 -- the only reason it's not biting Gecko is ... because we're apparently not following the spec for window.open properly in terms of how the navigation works.

But the upshot is that as the current spec is written if an iframe sandboxed with allow-popups-to-escape-sandbox and allow-popups but without allow-top-navigation does window.open(someurl) then the navigation should fail, because the iframe is not the "one permitted sandboxed navigator" for the new window and hence isn't allowed to navigate it.

@mikewest Please take a look. How is this working in Chrome right now?