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,
      );
      simulateMouseMove(null, firstTarget);
      expect(ops).toEqual(['enter parent']);
      ops = [];
      simulateMouseMove(firstTarget, secondTarget);
      expect(ops).toEqual([
        
        'enter portal',
      ]);
      ops = [];
      simulateMouseMove(secondTarget, thirdTarget);
      expect(ops).toEqual([
        'leave portal',
        'leave parent', 
      ]);
    } finally {
      document.body.removeChild(portalContainer);
    }
  });
  it('should throw on bad createPortal argument', () => {
    expect(() => {
      ReactDOM.createPortal(<div>portal</div>, null);
    }).toThrow('Target container is not a DOM element.');
    expect(() => {
      ReactDOM.createPortal(<div>portal</div>, document.createTextNode('hi'));
    }).toThrow('Target container is not a DOM element.');
  });
  it('should warn for non-functional event listeners', () => {
    class Example extends React.Component {