eat mouse wheel events destimed for map zoom

stop propagation, prevent default

frontend/src/Main.elm

@@ -404,12 +404,19 @@ canvas centre zoom width height track =
 portalWidth = 600
 portalHeight = 600
 
-wheelDecoder : D.Decoder Msg
-wheelDecoder =
-    D.map3 MapZoomWheel
-        (D.field "deltaX" D.float)
-        (D.field "deltaY" D.float)
-        (D.field "deltaMode" D.int)
+withSwallowing m =
+    { message = m
+    , stopPropagation = True
+    , preventDefault = True
+    }
+
+mapWheelDecoder =
+    D.map withSwallowing
+        (D.map3 MapZoomWheel
+             (D.field "deltaX" D.float)
+             (D.field "deltaY" D.float)
+             (D.field "deltaMode" D.int))
+
 
 viewDiv : Model -> Html Msg
 viewDiv model =
@@ -419,7 +426,7 @@ viewDiv model =
            , style "column-gap" "15px"
            ]
         [ div [ style "display" "flex"
-              , on "wheel" wheelDecoder
+              , Html.Events.custom "wheel" mapWheelDecoder
               , style "flex-direction" "column"]
               [ div [ style "width" (px portalWidth)
                     , style "height" (px portalHeight)