matroska

Explanation:

The flattened DOM looks like this: 
...
  < grand-father >
    #shadowRoot
      < the-father >
        #shadowRoot
          < the-child >
            #shadowRoot
              < slot#child >
                < slot#father >
                  < slot#grandFather >
                    matroska
Constructing this DOM produces six logs: 
THE-CHILD:      SLOT#father       =>
THE-FATHER:     SLOT#father       =>
THE-CHILD:      SLOT#child        =>
THE-CHILD:      SLOT#grandFather  => matroska
THE-FATHER:     SLOT#grandFather  => matroska
GRAND-FATHER:   SLOT#grandFather  => matroska
A view of the DOM in which the three slotchange events are executed. 
                         in-complete DOM                              completed DOM
  < grand-father >                           § < grand-father >                     _
    #shadowRoot                              §   #shadowRoot                         *6
      < the-father >                   _     §     < the-father >                   |
        #shadowRoot                    *2    §       #shadowRoot                     *5
          < the-child >                |  _  §         < the-child >                |
            #shadowRoot                *1 *3 §           #shadowRoot                 *4
              < slot#child >           |  ^  §             < slot#child >           |
                < slot#father >        ^     §               < slot#father >        |
                  < slot#grandFather >       §                 < slot#grandFather > ^
                    ""                       §                   matroska
  1. When log 1, 2, and 3 run, the DOM is in a temporary state. The grand-father element is created, but it does not have any child nodes yet: the "matroska" text node is not yet registered as its child. This means that when the log functions run assignedNodes({flatten: true}), it is empty.
  2. When log 4, 5, and 6 run, the DOM is completed. The "matroska" text node is now registered as a child of the grand-father host element, and the assignedNodes({flatten: true}) return the anticipated result "matroska".