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

leaveEventType,
    eventTypePrefix + 'leave',
    from,
    nativeEvent,
    nativeEventTarget,
    eventInterface,
  );
  leave.target = fromNode;
  leave.relatedTarget = toNode;
  let enter = new SyntheticEvent(
    enterEventType,
    eventTypePrefix + 'enter',
    to,
    nativeEvent,
    nativeEventTarget,
    eventInterface,
  );
  enter.target = toNode;
  enter.relatedTarget = fromNode;
  
  
  
  const nativeTargetInst = getClosestInstanceFromNode((nativeEventTarget: any));
  if (nativeTargetInst !== targetInst) {
    enter = null;
  }
  accumulateEnterLeaveTwoPhaseListeners(dispatchQueue, leave, enter, from, to);
}
export {registerEvents, extractEvents};