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

static childContextTypes = {
        foo: PropTypes.string.isRequired,
        getFoo: PropTypes.func.isRequired,
      };
      getChildContext() {
        return {
          foo: this.props.bar,
          getFoo: () => this.props.bar,
        };
      }
      render() {
        return ReactDOM.createPortal(<Component />, portalContainer);
      }
    }
    ReactDOM.render(<Parent bar="initial" />, container);
    expect(portalContainer.innerHTML).toBe('<div>initial-initial</div>');
    expect(container.innerHTML).toBe('');
    ReactDOM.render(<Parent bar="changed" />, container);
    expect(portalContainer.innerHTML).toBe('<div>changed-changed</div>');
    expect(container.innerHTML).toBe('');
  });
  it('findDOMNode should find dom element after expanding a fragment', () => {
    class MyNode extends React.Component {