partial from render field

app.re

let x = List.hd(List.rev(self.state.nav.position));
let x' = List.hd(self.state.nav.position);

...

<nav
  className=(self.state.nav.isOpen ? "active" : "")
  onClick=(event => ReactEventRe.Mouse.stopPropagation(event))
  style=(
    self.state.nav.isSwiping^ ?
      ReactDOMRe.Style.make(
        ~transform=
          "translateX("
          ++ string_of_float(x' -. x > 0.0 ? 0.0 : x' -. x)
          ++ "px)",
        ~transition="none",
        ()
      ) :
      ReactDOMRe.Style.make()
  )