elm - height of element in click handler

View.elm

import DOM -- From debois/elm-dom

view state config =
    div [ class "main" ]
        [ div [ class "nav"]
            [ button
                [ myClickHandler state config] -- When clicking on the button I want to get the height of element with class "displayBlock"
                [ text "MyButton" ]
                div
                [ class "displayNone" ]
                [ div [ class "displayBlock" ]  -- This is the element I need the height of. Works most of the time BUT !
                    [ p [] [ text "Some text" ]
                    , p [] [ text "Some more text" ]
                    ]
                ]
            ]
        , div [ class "content" ]
            [ div [ class "form" ] renderForm  -- BUT Sometimes this is the element I get the height of and I'm struggling to understand why
            , div [ class "whatever"] renderSomethingElse
            ]
        ]


myClickHandler state { toMsg } =
    heightDecoder
        |> Json.andThen
            (\v -> Json.succeed <| toMsg <| updState state v)  -- details left of but it's used in a larger context to initiate an animation
        |> on "click"


-- This fella works as I would expect most of the times, but sometimes it get's the wrong element... what's UP I WONDER ?
heightDecoder : Json.Decoder Float
heightDecoder =
    DOM.target <|
        DOM.parentElement <|
            DOM.nextSibling <|
                DOM.childNode 0 <|
                    DOM.offsetHeight