No background-color on <body> when display:flex.

In an fresh create-react-app, if:
`
body {
    background-color: #000000;
    display: flex;
}
`

There is no background color applied, until page refresh.

Somehow, It doesn't happen while render() includes an image tag, e.g. `<img src="/" />`.

I'm seeing the issue in Chrome 64, but not reproducing in Edge nor Firefox latest.