In the example at , an IntersectionObserver is used inside an iFrame (the jsFiddle iframe, in this case). You can observe that the intersectionRect reports something like left=846,top=517. According to the spec, this shouldn't be the case: > 6. Map intersectionRect to the coordinate space of the viewport of the Document containing the target. So I'd expect the intersectionRect to be relative to the document inside the iframe, thus reporting left=8,top=8. This works fine in Chrome, Safari, and Edge. : https://jsfiddle.net/8f9whxbk/11/ : https://w3c.github.io/IntersectionObserver/#calculate-intersection-rect-algo
I feel like this could have been done in bug 1359318, but I couldn't confirm this patch actually changed anything in older versions with mozregression. Since :tobytailor isn't working with us anymore, I figured it might be best just to open a new bug.
See Also: → 1359318
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