wayland plugin should distinguish touchpads from mice, etc.

registerInputDevice() is called from QWaylandInputDevice::seat_capabilities(uint32_t caps) ; in that context we have so far not been able to get enough information about the device capabilities etc. The seat capabilities don't include very many flags:

enum wl_seat_capability {
	/**
	 * the seat has pointer devices
	 */
	WL_SEAT_CAPABILITY_POINTER = 1,
	/**
	 * the seat has one or more keyboards
	 */
	WL_SEAT_CAPABILITY_KEYBOARD = 2,
	/**
	 * the seat has touch devices
	 */
	WL_SEAT_CAPABILITY_TOUCH = 4,
};

I left a TODO comment in https://codereview.qt-project.org/c/qt/qtwayland/+/301231 about this, but I still don't understand if there's anything in wayland protocol that lets us distinguish individual devices and their capabilities. I think surely such a feature must be in demand, not only from us.

WheelHandler's ctor has
setAcceptedDevices(QInputDevice::DeviceType::Mouse);
so since QWaylandInputDevice::seat_capabilities() assumes that it's a touchpad (because if you have a laptop touchpad, you want to be able to use native gestures), you often have to override WheelHandler's acceptedDevices to PointerDevice.Mouse | PointerDevice.TouchPad in order to get it to react. This is really not ideal.

Furthermore, we'd like QInputDevice::devices() to provide as complete a list as possible.

Wayland at this point is behaving just as badly as macOS: we can't get the devices; perhaps we could see at runtime that touchpad-like events are happening, but I hope we can eventually do better than that, even if it depends on some Wayland extension that is not yet written.