sandbox="allow-popups-to-escape-sandbox" was added to HTML standard in https://github.com/whatwg/html/pull/14

Some tests were added to wpt in https://github.com/w3c/web-platform-tests/pull/3905

This test gives interesting results:

http://web-platform.test:8000/html/semantics/embedded-content/the-iframe-element/iframe_sandbox_popups_nonescaping-3.html

The iframe opens an about:blank popup and tries to navigate it, which throws in Chrome/56.0.2924.59 but succeeds in 58.0.2997.0 canary and Gecko. I have not tried to follow exactly what the standard requires here, so not sure which behavior is correct. Please investigate. :-)

(This seems related to https://bugs.chromium.org/p/chromium/issues/detail?id=577330 but not quite since the test here just uses sandbox="allow-scripts allow-popups".)