Just the reducer field (solution 2)

app.re

[@bs.val] [@bs.scope "document"] external documentElement : Dom.element = "";

...

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"|}]; */
            ReactDOMRe.domElementToObj(documentElement)##style##overflow#="hidden";
          } else {
            /* [%bs.raw {| document.documentElement.style.overflow = ""|}]; */
            ReactDOMRe.domElementToObj(documentElement)##style##overflow#="";
          }
      )
    )
  },