some field of getViewport returns 0 instead of the actual value

[lilpacy]

We experienced that y field of getViewport return value is 0 instead of the actual value.

This is our code.

window.onpopstate = () => {
  console.log(window.pageYOffset);
  app.ports.onPopState.send(location.href);
};

        PopState here ->
            let
                _ =
                    Debug.log "PopState" here
            in
            ( model
            ,  Task.perform GotViewPort Browser.Dom.getViewport
            )

        GotViewPort viewport ->
            let
                _ =
                    Debug.log "vp" viewport
            ....

getViewport of Elm wraps window.pageYOffset of Javascript, but is it wrong?