type
  TMyComboBox = class(StdCtrls.TComboBox)
  private
    procedure WMLBUTTONDOWN(var msg: TMessage); message WM_LBUTTONDOWN;
end; 



type
  TfmTemplateOfInventory = class(TForm)
//....

  private
    procedure WndProc(var Message: TMessage); override;

var
   perSenderChild: TObject
   MousePoint: TPoint; 
end;


//тут получаем координаты курсора на форме 
procedure TfmTemplateOfInventory.WndProc(var Message: TMessage);
var
  WndHnd: THandle;
begin
  with Message do
    case Msg of

      CM_MOUSELEAVE:
        begin
          GetCursorPos(MousePoint);
          WndHnd := WindowFromPoint(MousePoint);
        end;
    end;
  inherited WndProc(Message);
end;


procedure TMyComboBox.WMLBUTTONDOWN(var msg: TMessage);
begin
inherited;
  case msg.Msg of
    WM_LBUTTONDOWN:
      begin
      fmTemplateOfInventory.StretchHandle1.Detach;
      fmTemplateOfInventory.StretchHandle1.Attach(FindDragTarget(MousePoint,true));
      perSenderChild:=FindDragTarget(MousePoint,true);
      end
  end;
end;