ContextMenuStrip not highlighting Items according to mouse movement

Question

I have a ContextMenuStrip that I am showing programmatically in response to a button being clicked. Everything works as expected, except that the Items in the menu do not respond to being moused over. Whether the mouse button is up or down, mousing over the menu has no visible effect, and releasing the mouse button does not select an Item, as expected. Performing a full click on an Item does still activate the Item, however.

Here's my code for showing the ContextMenuStrip:

Protected Overrides Sub OnMouseDown(ByVal e As System.Windows.Forms.MouseEventArgs)
  MyBase.OnMouseDown(e)
  If Enabled Then
    m_MouseDown = True
    If m_State > ButtonState.MousePressed Then
      m_State = ButtonState.MousePressed
    End If
    Invalidate()
    If DropDown IsNot Nothing AndAlso DropDown.Items.Count > 0 Then
      If ShowMenu Then
        ShowMenu = False
      ElseIf arrowRect.Contains(PointToClient(MousePosition)) Then
        ShowMenu = True
        m_MouseHeldWhileOpened = True
        DropDown.Capture = True
      End If
    End If
  End If
End Sub

Protected Property ShowMenu() As Boolean
  Get
    Return m_showMenu
  End Get
  Set(ByVal value As Boolean)
    If value <> m_showMenu Then
      m_ShowMenu = value
      If m_ShowMenu Then
        m_DropDown.Show(Me, GetDropDownSpawnPoint, DropDownDirection)
        m_State = ButtonState.MenuUp
        If m_DropDown.ClientRectangle.Contains(PointToScreen(MousePosition)) Then
          m_DropDown.Capture = True
        End If
      Else
        m_DropDown.Close()
        ElevateState()
      End If
    End If
  End Set
End Property

I have tried a number of different ideas to get the menu to respond correctly, some of which attempts are still evident in the code here. If any help would be forthcoming, it would be greatly appreciated - nobody else on Google seems to have experienced this problem.

Thank you in advance for any help you can offer.