Okay, I can see the behavior in Safari, and it's because the browser is not synchronizing the handling of the scrolling and the JS main thread. Most browsers only do that on the main window/body. So when you scroll, the browser instantly renders the content in that new position, THEN the JS runs and makes the necessary adjustment to make it appear fixed, so you see a judder. I am not aware of any way to force the browser to keep those things synchronized. If anyone else does, I'm all ears. 
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