Re-nominating for Fission based on the discussion in bug 1699846 comment 6. (Please let me know if I've misunderstood.) I'm not super familiar with this code, but it looks like the (only?) consumer of frame visbility info is related to [decoding images](https://searchfox.org/mozilla-central/rev/6d7c3099471817a7f9b0d6aac7a405cf6b1cd153/layout/base/PresShell.cpp#11006), so I think the failure mode here is, perhaps, that in OOP iframes, our heuristics to decode images when the enter the iframe's displayport may not kick in, and images may only get decoded once they enter the viewport? (Timothy, you might be able to confirm this.)
Bug 1649367 Comment 6 Edit History
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Re-nominating for Fission based on the discussion in bug 1699846 comment 6. (Please let me know if I've misunderstood.) I'm not super familiar with this code, but it looks like the (only?) consumer of frame visbility info is related to [decoding images](https://searchfox.org/mozilla-central/rev/6d7c3099471817a7f9b0d6aac7a405cf6b1cd153/layout/base/PresShell.cpp#11006), so I think the failure mode here is, perhaps, that in OOP iframes, our heuristics to decode images when they enter the iframe's displayport may not kick in, and images may only get decoded once they enter the viewport? (Timothy, you might be able to confirm this.)