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

}
      if (to) {
        to.dispatchEvent(
          new MouseEvent('mouseover', {
            bubbles: true,
            cancelable: true,
            relatedTarget: from,
          }),
        );
      }
    }
    try {
      ReactDOM.render(
        <div>
          <div
            onMouseEnter={() => ops.push('enter parent')}
            onMouseLeave={() => ops.push('leave parent')}>
            <div ref={n => (firstTarget = n)} />
            {ReactDOM.createPortal(
              <div
                onMouseEnter={() => ops.push('enter portal')}
                onMouseLeave={() => ops.push('leave portal')}
                ref={n => (secondTarget = n)}>
                portal
              </div>,
              portalContainer,
            )}
          </div>
          <div ref={n => (thirdTarget = n)} />
        </div>,