rsnext/test/integration/app-document/pages/_app.js

import App from 'next/app'
import React from 'react'
import { setState } from '../shared-module'

setState(typeof window === 'undefined' ? 'UPDATED' : 'UPDATED CLIENT')

class Layout extends React.Component {
  state = {
    random: false,
  }

  componentDidMount() {
    this.setState({ random: Math.random() })
  }

  render() {
    const { children } = this.props
    const { random } = this.state
    return (
      <div>
        <p id="hello-app">Hello App</p>
        <p id="hello-hmr">Hello HMR</p>
        <p id="random-number">{random}</p>
        {children}
      </div>
    )
  }
}

export default class MyApp extends App {
  static async getInitialProps({ Component, router, ctx }) {
    // throw _app GIP err here
    let pageProps = {}

    if (Component.getInitialProps) {
      pageProps = await Component.getInitialProps(ctx)
    }

    return { pageProps }
  }

  render() {
    const { Component, pageProps } = this.props
    return (
      <Layout>
        <Component {...pageProps} />
      </Layout>
    )
  }
}