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

render() {
        return this.props.children;
      }
    }
    class Fragment extends React.Component {
      render() {
        return [
          <Wrapper key="a">
            <div />
          </Wrapper>,
          <span key="b" />,
        ];
      }
    }
    let instance = null;
    ReactDOM.render(<Fragment ref={ref => (instance = ref)} />, container);
    expect(container.childNodes.length).toBe(2);
    const firstNode = ReactDOM.findDOMNode(instance);
    expect(firstNode).toBe(container.firstChild);
    expect(firstNode.tagName).toBe('DIV');
  });
  it('finds the first child even when first child renders null', () => {
    class NullComponent extends React.Component {
      render() {
        return null;
      }
    }
    class Fragment extends React.Component {