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This test shows that the mousewheel events generated in WebKit for a
"throw" trackpad gesture on Mac OS X are jittery (filed in Bugzilla as
https://bugs.webkit.org/show_bug.cgi?id=81040). Do a quick two-finger swipe
in the box on the left. You'll see that there's a lot of jitter in the
individual events, even well after the fingers have left the trackpad. On top
of the low-level jitter, there are larger-scale changes in slope that make
the overall curvature non-smooth. This causes "jumpiness" in the events that
makes things like virtualized scrolling look noticeably worse than native
scrolling.