Uses the Validation.ErrorTemplate to create a popup error indicator on a WPF TextBox control

WpfPopupError.xaml

        <Style TargetType="{x:Type TextBox}">
            <Setter Property="Validation.ErrorTemplate">
                <Setter.Value>
                    <ControlTemplate>
                        <Grid>
                            <Border BorderBrush="#FFCB2E2E" BorderThickness="1" Background="#11FF0000" IsHitTestVisible="False" x:Name="errorBorder"/>
                            <AdornedElementPlaceholder x:Name="placeholder" />
                            <Popup AllowsTransparency="True" HorizontalAlignment="Right" HorizontalOffset="0" VerticalOffset="0" PopupAnimation="Fade" Placement="Right" 
                                   PlacementTarget="{Binding ElementName=errorBorder}" IsOpen="{Binding ElementName=placeholder, Path=AdornedElement.IsFocused, Mode=OneWay}">
                                <StackPanel Orientation="Horizontal">
                                    <Polygon  VerticalAlignment="Center" Points="0,4 4,0 4,8" Fill="#FFCB2E2E" Stretch="Fill" Stroke="#FFCB2E2E"
                                      StrokeThickness="2" />
                                    <Border Background="#FFCB2E2E" CornerRadius="4" Padding="4">
                                        <TextBlock HorizontalAlignment="Center" Foreground="White" FontWeight="Bold" Margin="2,0,0,0"
                                                   Text="{Binding ElementName=placeholder, Path=AdornedElement.ToolTip, Mode=OneWay}" />
                                    </Border>
                                </StackPanel>
                            </Popup>
                        </Grid>
                    </ControlTemplate>
                </Setter.Value>
            </Setter>
            <Style.Triggers>
                <Trigger Property="Validation.HasError" Value="True">
                    <Setter Property="ToolTip" Value="{Binding RelativeSource={RelativeSource Self}, Path=(Validation.Errors)[0].ErrorContent}" />
                </Trigger>
            </Style.Triggers>
        </Style>

It looks nice. Sadly, the problem with popups is that they retain their original position when the containing window is moved or resized.

to solve this, you have to create a custom control, that inherits from window, so then you can move or resize