Reproducible Demo for https://github.com/facebook/create-react-app/issues/6399

App.js

import React, { Component } from "react";

class App extends Component {
  state = { height: "*loading*" };

  componentDidMount() {
    this.setState({
      height: this.wrapper.offsetHeight
    });
  }

  render() {
    return (
      <div
        ref={obj => {
          this.wrapper = obj;
        }}
      >
        My height is {this.state.height}
      </div>
    );
  }
}

export default App;

index.css

html, body {
  height: 100%;
  margin: 0;
}
div {
  display: block;
  height: 100%;
  background-color: red;
}

index.js

import React from 'react';
import ReactDOM from 'react-dom';
import App from './App';

import './index.css'

ReactDOM.render(<App />, document.getElementById('root'));