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
THE-FATHER   SLOT#grandFather
GRAND-FATHER SLOT#grandFather
  1. Log 1 and 2 are the same slotchange event, ie. slot#grandFather being transposed to slot#father.
  2. Log 3 is the innermost slotchange event, ie. slot#father being transposed to slot#child.
  3. Log 4, 5, and 6 is the final slotchange event, ie. "matroska" being transposed to slot#grandFather.
  4. The slotchange listener are triggered: 3 x the-child, 2 x the-father, 1 x grand-father. the-child web component has principally no way of knowing how many times its slotchange listener will be triggered during construction. 
...
  < grand-father >                                 _
    #shadowRoot                                    *6
      < the-father >                      _        |
        #shadowRoot                       *2       *5
          < the-child >                   |    _   |
            #shadowRoot                   *1   *3  *4
              < slot#child >              |    ^   |
                < slot#father >           ^        |
                  < slot#grandFather >             ^
                    matroska