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 (inst !== null) {
        return;
      }
    }
  }
  if (!isOutEvent && !isOverEvent) {
    
    return;
  }
  let win;
  
  if ((nativeEventTarget: any).window === nativeEventTarget) {
    
    win = nativeEventTarget;
  } else {
    
    const doc = (nativeEventTarget: any).ownerDocument;
    if (doc) {
      win = doc.defaultView || doc.parentWindow;
    } else {
      win = window;
    }
  }
  let from;
  let to;
  if (isOutEvent) {
    const related = nativeEvent.relatedTarget || (nativeEvent: any).toElement;
    from = targetInst;
    to = related ? getClosestInstanceFromNode((related: any)) : null;
    if (to !== null) {