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

const hideOnFocus = () => {
        
        ReactDOM.unmountComponentAtNode(container);
      };
      return (
        <button onFocus={hideOnFocus} ref={ref}>
          old
        </button>
      );
    }
    function NewApp() {
      return <button ref={ref}>new</button>;
    }
    ReactDOM.render(<OldApp />, container);
    ref.current.focus();
    ReactDOM.render(<NewApp />, container);
    
    
    ref.current.focus();
    expect(container.textContent).toBe('new');
  });
});