onScroll 
  fun onScroll(connectFlags: <Error class: unknown class> = ConnectFlags(0u), handler: (<Error class: unknown class>, <Error class: unknown class>) -> Boolean): <Error class: unknown class>(source)
Signals that the widget should scroll by the amount specified by @dx and @dy.
For the representation unit of the deltas, see method@Gtk.EventControllerScroll.get_unit.
Parameters
connectFlags 
a combination of ConnectFlags
handler
the Callback to connect. Params: dx X delta; dy Y delta. Returns true if the scroll event was handled, false otherwise.