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.
const portalContainer = document.createElement('div');
    ReactDOM.render(
      <div>{ReactDOM.createPortal(<div>portal</div>, portalContainer)}</div>,
      container,
    );
    expect(portalContainer.innerHTML).toBe('<div>portal</div>');
    expect(container.innerHTML).toBe('<div></div>');
    ReactDOM.unmountComponentAtNode(container);
    expect(portalContainer.innerHTML).toBe('');
    expect(container.innerHTML).toBe('');
  });
  
  it('should support unstable_createPortal alias', () => {
    const portalContainer = document.createElement('div');
    expect(() =>
      ReactDOM.render(
        <div>
          {ReactDOM.unstable_createPortal(<div>portal</div>, portalContainer)}
        </div>,
        container,
      ),
    ).toLowPriorityWarnDev(
      'The ReactDOM.unstable_createPortal() alias has been deprecated, ' +
        'and will be removed in React 17+. Update your code to use ' +
        'ReactDOM.createPortal() instead. It has the exact same API, ' +
        'but without the "unstable_" prefix.',
      {withoutStack: true},
    );
    expect(portalContainer.innerHTML).toBe('<div>portal</div>');
    expect(container.innerHTML).toBe('<div></div>');