WPF DataGridComboBoxColumn style

I use ComboboxColumn in DataGrid which works fine.

However there is a style issue:
When cursor is not on the column, the ComboboxColumn looks like a textboxColumn. Once I click it, a tiny arrow on the right corner displays so it looks like a combobox.

I want it to display like a combobox even when it is not clicked or focused. I feel it isElementStyle, but not sure what to change to exactly. Does someone know how to do this?

xmlns:dg="http://schemas.microsoft.com/wpf/2008/toolkit"

                   <dg:DataGridComboBoxColumnWidth="200"x:Name="column"Header="Column"
    SelectedValueBinding="{Binding Path=RelParams.ColumnName, UpdateSourceTrigger=PropertyChanged,
    diagnostics:PresentationTraceSources.TraceLevel= High}"
    DisplayMemberPath="cName"
    SelectedValuePath="cName">

    <dg:DataGridComboBoxColumn.ElementStyle>
        <StyleTargetType="ComboBox">
            <Setter Property="ItemsSource" Value="{Binding RelInfo.Columns}"/>
        </Style>
    </dg:DataGridComboBoxColumn.ElementStyle>
    <dg:DataGridComboBoxColumn.EditingElementStyle>
        <StyleTargetType="ComboBox">
            <Setter Property="ItemsSource" Value="{Binding RelInfo.Columns}"/>
        </Style>
    </dg:DataGridComboBoxColumn.EditingElementStyle>

</dg:DataGridComboBoxColumn>

Peace & Joy