React version: 0.0.0-f77c7b9d7
Browser: Chrome Version 84.0.4147.105 (Official Build) (64-bit)
OS: Ubuntu 18.04.4 LTS

## Steps To Reproduce

1. move mouse over button

Link to code example:

https://codesandbox.io/s/mouseenter-in-reactnext-ibld3?file=/src/Demo.js

## The current behavior

`onMouseEnter` fires twice (sometimes it doesn't).

![video capture of repro steps](https://i.ibb.co/XzdQF68/react-next-mouseenter-twice.gif)

## The expected behavior

It fires only once. Same repro with `react@16.13.1`: https://codesandbox.io/s/mouseenter-in-react16131-9sr7b?file=/src/Demo.js

expect(portalContainer.innerHTML).toBe('');
      expect(container.innerHTML).toBe('');
    });
  }
  it('should render many portals', () => {
    const portalContainer1 = document.createElement('div');
    const portalContainer2 = document.createElement('div');
    const ops = [];
    class Child extends React.Component {
      componentDidMount() {
        ops.push(`${this.props.name} componentDidMount`);
      }
      componentDidUpdate() {
        ops.push(`${this.props.name} componentDidUpdate`);
      }
      componentWillUnmount() {
        ops.push(`${this.props.name} componentWillUnmount`);
      }
      render() {
        return <div>{this.props.name}</div>;
      }
    }
    class Parent extends React.Component {
      componentDidMount() {
        ops.push(`Parent:${this.props.step} componentDidMount`);
      }
      componentDidUpdate() {
        ops.push(`Parent:${this.props.step} componentDidUpdate`);
      }
      componentWillUnmount() {