index.html

  <script type ="text/javascript">
    var app = Elm.fullscreen(Elm.Main, {
      lastItemVisible: false
    });

   window.onscroll = function () {
     var wrapper = document.getElementsByClassName("cset-list")[0];
     var lastItem = wrapper.childNodes[wrapper.childNodes.length - 1];

     console.log("last item offset: " + wrapper.childNodes.length);

     if (isElementInViewport(lastItem)) {
       app.ports.lastItemVisible.send(true);
       console.log("sent true: " + lastItem);
      } else {
       app.ports.lastItemVisible.send(false);
       console.log("sent false: " + lastItem);
      }
   };
  </script>

Main.elm

type alias Model = 
  { 
    -- bunch of fields
      , isLastItemVisible : Bool
  }
  
type Action
  = None
  -- bunch of actions
 | UpdateLastItemVisible Bool
 
 update: Action -> Model -> (Model, Effects Action)
update action model = 
  case action of
      -- bunch of actions
     UpdateLastItemVisible b ->
         ( { model | isLastItemVisible = b }, Effects.none )

view: Address Action -> Model -> Html
view address model =
   -- bunch of html things
   [ h2 [] [ text (if model.isLastItemVisible then "Visible" else "Nope") ] ]
   
port lastItemVisible : Signal Bool

lastItemVisibleActions: Signal Action
lastItemVisibleActions =
  Signal.map (\b -> UpdateLastItemVisible b) lastItemVisible