You will also have to explicitly set the column widths within the embedded grid in the GridListControl. Here is code that worked for me in version 2.1.0.9.
Private Sub gridControl1_CurrentCellShowingDropDown(ByVal sender As Object, ByVal e As Syncfusion.Windows.Forms.Grid.GridCurrentCellShowingDropDownEventArgs)
Dim cc As GridCurrentCell = Me.gridControl1.CurrentCell
If Me.gridControl1(cc.RowIndex, cc.ColIndex).CellType = "GridListControl" Then
Dim cr As GridDropDownGridListControlCellRenderer = cc.Renderer
''cr.ListControlPart.AutoSizeColumns = False
cr.ListControlPart.Grid.ColWidths(1) = Me.gridControl1.ColWidths(1) - 1
cr.ListControlPart.Grid.ColWidths(2) = Me.gridControl1.ColWidths(2) - 1
e.Size = New Size(Me.gridControl1.ColWidths(1) + Me.gridControl1.ColWidths(1), e.Size.Height)
cr.ListControlPart.Grid.HScrollBehavior = GridScrollbarMode.Disabled
End If
End Sub