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.
expect(container.innerHTML).toBe('');
    expect(ops).toEqual([
      'Parent:b componentWillUnmount',
      'normal[0]:b componentWillUnmount',
      'portal1[0]:b componentWillUnmount',
      'normal[1]:b componentWillUnmount',
      'portal2[0]:b componentWillUnmount',
      'portal2[1]:b componentWillUnmount',
    ]);
  });
  it('should render nested portals', () => {
    const portalContainer1 = document.createElement('div');
    const portalContainer2 = document.createElement('div');
    const portalContainer3 = document.createElement('div');
    ReactDOM.render(
      [
        <div key="a">normal[0]</div>,
        ReactDOM.createPortal(
          [
            <div key="b">portal1[0]</div>,
            ReactDOM.createPortal(
              <div key="c">portal2[0]</div>,
              portalContainer2,
            ),
            ReactDOM.createPortal(
              <div key="d">portal3[0]</div>,
              portalContainer3,
            ),
            <div key="e">portal1[1]</div>,
          ],