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

return null;
      }
    }
    let inst;
    ReactDOM.render([<Example key="a" ref={n => (inst = n)} />], container);
    const node = container.firstChild;
    expect(node.tagName).toEqual('DIV');
    click();
    expect(ops).toEqual(['A']);
    ops = [];
    
    inst.flip();
    click();
    expect(ops).toEqual(['B']);
    ops = [];
    
    inst.tick();
    click();
    expect(ops).toEqual(['B']);
    ops = [];
    
    
    
    
    ReactDOM.render(
      [<Example key="a" forceA={true} />, <Click key="b" />],
      container,