code: async function(row, col, event) {
          if (this.isTreeCol(col)) {
              let isShift = false;
              if (event.primitiveEvent.detail.primitiveEvent) {
                  isShift = !!event.primitiveEvent.detail.primitiveEvent.shiftKey; // typecast to boolean
              }
              let is_expanded = await this._view.get_row_expanded(row);
              if (isShift) {
                  if (is_expanded) {
                      if (this.data[row][col].rowPath.length === 1) {
                          this._view.collapse(row);
                      } else {
                          this._view.set_depth(this.data[row][col].rowPath.length - 2);
                      }
                  } else {
                      this._view.set_depth(this.data[row][col].rowPath.length - 1);
                  }
              } else {
                  if (is_expanded) {
                      this._view.collapse(row);
                  } else {
                      this._view.expand(row);
                  }
              }
              let nrows = await this._view.num_rows();
              this.setDirty(nrows);
              this.grid.canvas.paintNow();
          }
      }
docs: Called when clicking on a row group expand