Extracted from https://github.com/facebook/react/issues/12643.

We're currently diff a host node in the complete phase even if it has bailed out in the begin phase. By itself, this just means we're doing a little bit more work than necessary. But it also has some observable effects. In particular, https://github.com/facebook/react/issues/13424 (which by itself would be very rare and probably not worth fixing) is made worse because we end up committing update to inputs from another component.

I have a fix for this in https://github.com/facebook/react/pull/13423. Filing this for posterity.
render() {
        return !this.props.flag
          ? [<div key="a" />]
          : [<span key="b" />, <div key="a" />];
      }
    }
    const myNodeA = ReactDOM.render(<MyNode />, container);
    const a = ReactDOM.findDOMNode(myNodeA);
    expect(a.tagName).toBe('DIV');
    const myNodeB = ReactDOM.render(<MyNode flag={true} />, container);
    expect(myNodeA === myNodeB).toBe(true);
    const b = ReactDOM.findDOMNode(myNodeB);
    expect(b.tagName).toBe('SPAN');
  });
  it('should bubble events from the portal to the parent', () => {
    const portalContainer = document.createElement('div');
    document.body.appendChild(portalContainer);
    try {
      const ops = [];
      let portal = null;
      ReactDOM.render(
        <div onClick={() => ops.push('parent clicked')}>
          {ReactDOM.createPortal(
            <div
              onClick={() => ops.push('portal clicked')}
              ref={n => (portal = n)}>
              portal
            </div>,
            portalContainer,