code: function layoutRealizedRange() {
                          if (that._groups.length === 0) {
                              return;
                          }
  
                          var realizedItemRange = that._getRealizationRange(),
                              len = tree.length,
                              i,
                              firstChangedGroup = site.groupIndexFromItemIndex(changedRange.firstIndex);
  
                          for (i = firstChangedGroup; i < len; i++) {
                              layoutGroupContent(i, realizedItemRange, true);
                              that._layoutGroup(i);
                          }
                      }
docs: Synchronously lays out: - Realized and unrealized group shells (header containers and items containers). This is needed so that each realized group will be positioned at the correct offset. - Realized items.