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

container.innerHTML = '<div>MEOW.</div>';
    expect(() => {
      expect(() =>
        ReactDOM.render(<div key="2">baz</div>, container),
      ).toErrorDev(
        'render(...): ' +
          'It looks like the React-rendered content of this container was ' +
          'removed without using React. This is not supported and will ' +
          'cause errors. Instead, call ReactDOM.unmountComponentAtNode ' +
          'to empty a container.',
        {withoutStack: true},
      );
    }).toThrowError();
  });
  it('should warn when doing an update to a container manually updated outside of React', () => {
    
    ReactDOM.render(<div>foo</div>, container);
    ReactDOM.render(<div>bar</div>, container);
    expect(container.innerHTML).toBe('<div>bar</div>');
    
    container.innerHTML = '<div>MEOW.</div>';
    expect(() =>
      ReactDOM.render(<div>baz</div>, container),
    ).toErrorDev(
      'render(...): ' +
        'It looks like the React-rendered content of this container was ' +
        'removed without using React. This is not supported and will ' +