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

'Parent:b componentWillUnmount',
      'normal[0]:b componentWillUnmount',
      'portal1[0]:b componentWillUnmount',
      'normal[1]:b componentWillUnmount',
      'portal2[0]:b componentWillUnmount',
      'portal2[1]:b componentWillUnmount',
    ]);
  });
  it('should render nested portals', () => {
    const portalContainer1 = document.createElement('div');
    const portalContainer2 = document.createElement('div');
    const portalContainer3 = document.createElement('div');
    ReactDOM.render(
      [
        <div key="a">normal[0]</div>,
        ReactDOM.createPortal(
          [
            <div key="b">portal1[0]</div>,
            ReactDOM.createPortal(
              <div key="c">portal2[0]</div>,
              portalContainer2,
            ),
            ReactDOM.createPortal(
              <div key="d">portal3[0]</div>,
              portalContainer3,
            ),
            <div key="e">portal1[1]</div>,
          ],
          portalContainer1,
        ),