determine if onvisibilitychange is a fingerprinting vector
In Firefox 56, Mozilla added support for onvisibilitychange. See: https://developer.mozilla.org/en-US/docs/Web/API/Document/onvisibilitychange https://bugzilla.mozilla.org/show_bug.cgi?id=1333912
We should think about whether this introduces any new fingerprinting risks. The following mentions a prerender state which we (mcs and brade) do not fully understand: https://w3c.github.io/page-visibility/#privacy-security