ManageScroll.js

import React from "react";
import { Location } from "@reach/router";

let scrollPositions = {};

class ManageScrollImpl extends React.Component {
  componentWillUnmount() {
    scrollPositions[this.props.location.key] = window.scrollY;
  }

  componentDidUpdate() {
    window.scrollTo(0, scrollPositions[this.props.location.key] || 0);
  }

  render() {
    return null;
  }
}

export default () => (
  <Location>
    {({ location }) => <ManageScrollImpl location={location} />}
  </Location>
);

Answered here.