Load daringrireball.net on trunk and do a slightly diagonal scroll swipe (or a J-shaped scroll). The vertical momentum stalls when the rubberband animation starts (and then you sometimes get a bit at the end).
<rdar://problem/86558157>
This probably regressed at https://trac.webkit.org/changeset/284136/webkit
Created attachment 449614 [details] Patch
Created attachment 449616 [details] Patch
Committed r288336 (246244@main): <https://commits.webkit.org/246244@main> All reviewed patches have been landed. Closing bug and clearing flags on attachment 449616 [details].