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(ops).toEqual(['B']);
    ops = [];
    
    click();
    expect(ops).toEqual(['A']);
  });
  it('should not crash encountering low-priority tree', () => {
    ReactDOM.render(
      <div hidden={true}>
        <div />
      </div>,
      container,
    );
  });
  it('should not warn when rendering into an empty container', () => {
    ReactDOM.render(<div>foo</div>, container);
    expect(container.innerHTML).toBe('<div>foo</div>');
    ReactDOM.render(null, container);
    expect(container.innerHTML).toBe('');
    ReactDOM.render(<div>bar</div>, container);
    expect(container.innerHTML).toBe('<div>bar</div>');
  });
  it('should warn when replacing a container which was manually updated outside of React', () => {
    
    ReactDOM.render(<div key="1">foo</div>, container);
    ReactDOM.render(<div key="1">bar</div>, container);
    expect(container.innerHTML).toBe('<div>bar</div>');