I want to change the border of a cell to red when there's an error. This sounds pretty basic.

My style is:

        <Style x:Key="cell_error" TargetType="{x:Type xcdg:DataCell}">
            <Setter Property="BorderBrush">
                <Setter.Value>
                    <SolidColorBrush Color="Red"/>
                </Setter.Value>
            </Setter>
            <Setter Property="BorderThickness" Value="1,1,1,1" />
        </Style>

I have a CellValidationRules applied to a data column so that the ValidationMode property is effective.

Problem:
If ValidationMode="CellEndingEdit", the cell border's color does not change to red on input error when the cell loses focus.

However, if my error style is set to change the background color to red, then background color DOES red on error when the cell loses focus.

Why am I able to change the background color, but not the border color?