A wheel turned: it scrolls (Shift turns a vertical wheel sideways; in a paged mode at a spread's edge it turns the spread, once per burst of wheel events), and with Ctrl or Cmd held it zooms about the pointer, as trackpad pinches arrive in browsers.
public void Wheel(double deltaX, double deltaY, WheelDeltaMode mode, InputModifiers modifiers, double x, double y)
The horizontal delta (positive scrolls right).
The vertical delta (positive scrolls down, or zooms out).
The deltas' unit.
The modifier keys held.
The pointer's x in device-independent pixels.
The pointer's y in device-independent pixels.
Spec #33 viewer capability 14. Zoom steps settle after SettleDelay.