CSP: 'no-external-navigation'?

Pamela Fox submitted this bug to Mozilla, but it's probably better
discussed here: https://bugzilla.mozilla.org/show_bug.cgi?id=1032310

We've talked briefly about similar concepts in the past in the context of
the next iteration of CSP; at first glance it seems like something that
might be useful in narrow use-cases, but that I'm worried will be abused to
keep folks on pages they don't particularly want to be on (see  what
happened with `window.onbeforeunload` modals).

Thoughts?

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Mike West <mkwst@google.com>
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