mouseEnter behaves like mouseOver when using ReactDOM.render() to mount a child element

<!-- Note: if the issue is about documentation or the website, please file it at: https://github.com/reactjs/reactjs.org/issues/new --> **Do you want to request a *feature* or report a *bug*?** Bug **What is the current behavior?** When using `ReactDOM.render()` to mount a child element, `mouseEnter` behaves like `mouseOver` (`mouseLeave` behaves like `mouseOut`). you can see the demo, and when my cursor moves between the red and blue blocks, it will repeatedly trigger mouseEnter and mouseLeave. https://codepen.io/sen-dream/pen/VwwvGbm **What is the expected behavior?** https://codepen.io/sen-dream/pen/WNNQgoy **Which versions of React, and which browser / OS are affected by this issue? Did this work in previous versions of React?** Testes with React 16.8 on Chrome/macOS. It didn't work in previous versions.