message: Add onDetailPanelShowingChanged callback
diff: @@ -834,6 +834,7 @@ export class MatrixDropdownRowModelBase
  public cells: Array<MatrixDropdownCell> = [];
  public showHideDetailPanelClick: any;
  + public onDetailPanelShowingChanged: () => void;
  constructor(data: IMatrixDropdownData, value: any) {
  this.data = data;
  @@ -892,6 +893,9 @@ export class MatrixDropdownRowModelBase
  if (!!this.data) {
  this.data.setIsDetailPanelShowing(this, val);
  }
  + if (!!this.onDetailPanelShowingChanged) {
  + this.onDetailPanelShowingChanged();
  + }
  }
  private showHideDetailPanel() {
  if (this.isDetailPanelShowing) {
  