So, apparently the problem is that `navigator.buildID` is cached, so the second time we access it, it returns whatever its value was the first time we accessed it. Somehow, accessing it through the sandbox manages to avoid the caching. But I think that, given there's clearly a behavior difference when accessing it through a Sandbox, the test should avoid doing that, and should do something like reloading the iframe instead. This seems to work: https://gist.github.com/be00506b969fb88fef6e642ac7dc16a8
Bug 1587013 Comment 4 Edit History
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So, apparently the problem is that `navigator.buildID` is cached, so the second time we access it, it returns whatever its value was the first time we accessed it. Somehow, accessing it through the sandbox manages to avoid the caching. But I think that, given there's clearly a behavior difference when accessing it through a Sandbox, the test should avoid doing that, and should do something like reloading the iframe instead. This seems to work: https://gist.github.com/5660ff71f40bd5bc4e8f442a2ee903bb