See http://bit.ly/2wCqNvc Facebook is doing 7 reflows when handling the load event. Is it possible for them to do better?
Facebook suggests that these reflows (from ViewportTrackingHelper) account for just 0.5% of our startup time in Chrome. Data does look suspiciously low to be fair. Facebook could mitigate this by using the same layout flush to check all viewport tracking but that's not a simple change. There's also the option of using IntersectionObserver which is now shipping.
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