It's unclear if we can do it and if so which kind of information from the top-level viewport we would expose to the cross-origin iframe. For Regular `IntersectionObserver` If we decide this is not an issue for percentages, then [this rect](https://searchfox.org/mozilla-central/rev/b2ccce862ef38d0d150fcac2b597f7f20091a0c7/dom/base/DOMIntersectionObserver.cpp#483) should be fixed and be the top level document rect instead.
Bug 1618676 Comment 0 Edit History
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It's unclear if we can do it and if so which kind of information from the top-level viewport we would expose to the cross-origin iframe. For Regular `IntersectionObserver` we don't apply root margins, because percentages would expose the size of the viewport (or that's my reading). But it seems that given the root margin is in our control we wouldn't be exposing so much to the third-party context. If we decide this is not an issue even for percentages, then [this rect](https://searchfox.org/mozilla-central/rev/b2ccce862ef38d0d150fcac2b597f7f20091a0c7/dom/base/DOMIntersectionObserver.cpp#483) should be fixed and be the top level document rect instead. That being said this is probably wontfix, just filing to capture the conversation me and Hiro just had about this.