Following Timothy's suggestion, I graphed scroll position vs. time when a fast fling approaches the edge of the page. The steep initial part of the graph, with the values above zero, is the fling approaching the edge of the page with high velocity. The remainder of the graph, with the values below zero, is the overscroll animation. The first few frames are smooth, then there's a sharp discontinuity, and then the rest of the animation is smooth.
Bug 1704659 Comment 6 Edit History
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Following Timothy's suggestion, I graphed scroll position vs. time when a fast fling approaches the edge of the page. The steep initial part of the graph, with the values above zero, is the fling approaching the edge of the page with high velocity. The remainder of the graph, with the values below zero, is the overscroll animation. The first few frames are smooth, then there's a sharp discontinuity (==> this presumably is the problem), and then the rest of the animation is smooth.