The WheelHandler should be usable for managing two-finger pan gestures on the Macbook trackpad. However, the wheel event returns Mouse as the device type both for trackpad events, as well as for actual wheel events coming from a connected mouse.
Attached example logs the same device type value regardless of whether the wheel event came from a mouse or the touchpad.
No other input handler seems capable of dealing with two finger drag on the touchpad (without a click to drag). So, the wheel events need to properly identify the source device in order to support both mouse wheels and trackpads. Limiting the WheelHandler to just TouchPad devices also doesn't work, since it apparently sees the trackpad as a mouse device.