The focal point (cursor, or finger midpoint) in viewport client coordinates.
The gesture's own scroll position for this level, in screen (zoomed) px.
The pan that rode along with this step — how far the fingers' midpoint moved on screen since the
previous step, in px, positive toward the right/bottom (so the content follows and scroll
decreases). 0 for a wheel, and on end.
start on the first change of a gesture, move while it runs, end once it is over — for a
wheel, after the stream has been quiet for a moment; for touch, on the fingers lifting. end
repeats the final level so the owner can round, snap and settle.
The new zoom factor (1 = 100 %), continuous and already clamped to the gesture's range.
One step of a zoom gesture (ctrl/⌘+wheel; pinch), requested on the viewport by the ScrollPane that owns the gesture and applied by whoever owns the zoom's model (a sheet element) — the viewport itself only relays it (see IViewport.requestZoom).