code: function layoutGroupContent(groupIndex, realizedItemRange, doRealizedRange) {
                          var group = that._groups[groupIndex],
                              firstChangedIndexInGroup = Math.max(0, changedRange.firstIndex - group.startIndex),
                              realizedItemRangeInGroup = that._rangeForGroup(group, realizedItemRange),
                              beforeRealizedRange;
  
                          if (doRealizedRange) {
                              group.layoutRealizedRange(firstChangedIndexInGroup, realizedItemRangeInGroup);
                          } else {
                              if (!realizedItemRangeInGroup) {
                                  beforeRealizedRange = (group.startIndex + group.count - 1 < realizedItemRange.firstIndex);
                              }
  
                              return group.layoutUnrealizedRange(firstChangedIndexInGroup, realizedItemRangeInGroup, beforeRealizedRange);
                          }
                      }
docs: When doRealizedRange is true, this function is synchronous and has no return value. When doRealizedRange is false, this function is asynchronous and returns a promise.