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

portal
            </div>,
            portalContainer,
          )}
        </div>,
        container,
      );
      expect(portal.tagName).toBe('DIV');
      portal.click();
      expect(ops).toEqual(['portal clicked', 'parent clicked']);
    } finally {
      document.body.removeChild(portalContainer);
    }
  });
  it('should not onMouseLeave when staying in the portal', () => {
    const portalContainer = document.createElement('div');
    document.body.appendChild(portalContainer);
    let ops = [];
    let firstTarget = null;
    let secondTarget = null;
    let thirdTarget = null;
    function simulateMouseMove(from, to) {
      if (from) {
        from.dispatchEvent(
          new MouseEvent('mouseout', {
            bubbles: true,
            cancelable: true,
            relatedTarget: to,
          }),
        );