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

constructor(props) {
        super(props);
        this.state = {
          show: true,
        };
      }
      render() {
        return (
          <div>
            {this.state.show && (
              <>
                {ReactDOM.createPortal(null, portalContainer)}
                <div>child</div>
              </>
            )}
            <div>parent</div>
          </div>
        );
      }
    }
    const instance = ReactDOM.render(<Wrapper />, container);
    expect(container.innerHTML).toBe(
      '<div><div>child</div><div>parent</div></div>',
    );
    instance.setState({show: false});
    expect(instance.state.show).toBe(false);
    expect(container.innerHTML).toBe('<div><div>parent</div></div>');
  });
  it('should keep track of namespace across portals (simple)', () => {