Just the reducer field (solution 1)

app.re

reducer: (action, state) =>
    switch action {
    | Navigate(_routeWithTitle) => ReasonReact.NoUpdate
    | ToggleMenu(isOpen) =>
      ReasonReact.UpdateWithSideEffects(
        {
          ...state,
          nav: {
            isOpen: isOpen
          }
        },
        (
          _self =>
            if (isOpen) {
              [%bs.raw
                {| document.documentElement.style.overflow = "hidden"|}
              ];
            } else {
              [%bs.raw {| document.documentElement.style.overflow = ""|}];
            }
        )
      )
    },