Created attachment 380363 [details] Repro HTML page Consider a web page trying to measure framerate by using the timestamps passed in to requestAnimationFrame callbacks. Per the event loop spec (https://html.spec.whatwg.org/multipage/webappapis.html#event-loop-processing-model) it seems that the timestamps should typically be ~16ms apart (and certainly >0ms apart), but on iOS Safari it's possible for them to be 0ms apart. Here is a sample page to demonstrate: https://bl.ocks.org/nolanlawson/raw/99773cc8edc4115e6ba156a503dd2c41/ You might have to refresh the page a few times to reproduce, but often there will be one rAF callback whose timestamp is exactly the same as the previous one, e.g.: 12 24 (+12) 34 (+10) 34 (+0) <-- exactly the same as the last one 43 (+9) So far this seems only reproducible on iOS Safari. I can repro on both iOS 12.4 and 13.0 (using a real iPhone for the first and a simulator for the second). I'm not sure if the issue is with the timestamp or if the rAF callbacks are truly firing on the same turn of the event loop.
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