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

state = {};
      componentDidMount() {
        document.addEventListener('click', this.update, true);
      }
      componentWillUnmount() {
        document.removeEventListener('click', this.update, true);
      }
      update = () => {
        
        
        
        
        
        this.setState({});
        
        
        
        
        
      };
      render() {
        return <div />;
      }
    }
    class Parent extends React.Component {