Need access to popupContainer element.

I am using rc-slider and I need to set the z-index for the element created by getContainer.
I also propose putting this logic inside componentDidMount() and clean up in componentWillUnmount.

Alternatively you could set the styling for the element within the portal. Right now it's a bit tricky to work with.

getContainer = () => {
  const { props } = this;
  const popupContainer = document.createElement('div');
  // Make sure default popup container will never cause scrollbar appearing
  // https://github.com/react-component/trigger/issues/41
  popupContainer.style.position = 'absolute';
  popupContainer.style.top = '0';
  popupContainer.style.left = '0';
  popupContainer.style.width = '100%';
  const mountNode = props.getPopupContainer ?
    props.getPopupContainer(findDOMNode(this)) : props.getDocument().body;
  mountNode.appendChild(popupContainer);
  return popupContainer;
}

Comienza localizando la implementación de getContainer en el código del trigger y lee cómo se crea y se elimina el contenedor del popup. El issue estará resuelto cuando los llamadores puedan acceder al contenedor creado o aplicar estilos sobre él, y el comportamiento de su ciclo de vida esté cubierto por las pruebas existentes relevantes.