Data Validation Multiple Selection - Keep Dropdown Box Open to Make Selections

My spreadsheet contains a column with data validation that I have inserted code (below) that will allow multiple selections. Is there additional code that will allow the drop down list to stay open while selecting the multiple selections. Currently, I have to select one and then go back and click the drop down arrow again in order make another selection. It would be ideal if the list would stay open during the multiple selections.

Any assistance is appreciated.

Margaret

Private Sub Worksheet_Change(ByVal Target As Range)
Dim rngDV As Range
Dim Oldval As String
Dim Newval As String
If Target.Count > 1 Then GoTo exitHandler

On Error Resume Next
Set rngDV = Cells.SpecialCells(xlCellTypeAllValidation)
On Error GoTo exitHandler

If rngDV Is Nothing Then GoTo exitHandler
If Intersect(Target, rngDV) Is Nothing Then
    'do nothing
Else
    Application.EnableEvents = False
    Newval = Target.Value
    Application.Undo
    Oldval = Target.Value
    Target.Value = Newval
    If Target.Column = 12 Then
      If Oldval = "" Then
      Else
        If Newval = "" Then
        Else
        Target.Value = Oldval & ", " & Newval
        End If
      End If
  End If
End If

exitHandler:
  Application.EnableEvents = True
End Sub