Currently, to support a continuous scroll device, WheelEventMac uses an SPI (wkGetWheelEventDeltas) (see http://trac.webkit.org/browser/trunk/WebCore/platform/mac/WheelEventMac.mm). This isn't necessary anymore in Leopard.
In Chromium, we're using CGEvents to get the data we need. See http://src.chromium.org/viewvc/chrome/trunk/src/webkit/glue/webinputevent_mac.mm .
WebKit should switch for two reasons:
- The use of SPIs is bad and should be reduced
- The measurement of scrolling units should agree across WebKit browsers. For example, the DOM scroll event should have non-accelerated ticks. We have that in Chromium. Does WebKit? It's hard to say as the SPI isn't documented.