TextBox TextTrimming

Try a style like this (I’ve added background colours to make the change obvious):

    <Style TargetType="TextBox">
      <Setter Property="Background" Value="Yellow" />
      <Style.Triggers>
        <DataTrigger Binding="{Binding IsKeyboardFocused, RelativeSource={RelativeSource Self}}" Value="false">
          <Setter Property="Template">
            <Setter.Value>
              <ControlTemplate TargetType="TextBox">
                <TextBlock Text="{TemplateBinding Text}"  TextTrimming="CharacterEllipsis" Background="Red" />
              </ControlTemplate>
            </Setter.Value>
          </Setter>
        </DataTrigger>
      </Style.Triggers>
    </Style>