procedure TformMain.ButtonToggleLayerGroupingClick(Sender: TObject);
begin
  // Iterate through all selected layers (except background):
  with ImageEnView1 do
  begin
    LockUpdate();
    try
      var ThisGroupIndex := Random(Max_Int);
      for var i := 1 to LayersCount - 1 do
      begin
        if Layers[i].Selected then
        begin
          if Layers[i].GroupIndex = 0 then // if Layer is UNGROUPED
            Layers[i].GroupIndex := ThisGroupIndex
          else // if Layer is GROUPED
            Layers[i].GroupIndex := 0;
        end;
      end;
    finally
      UnlockUpdate();
    end;
  end;
end;