3rdpartylibs/fastreports.wpf/FastReport.Forms.WPF/CustomControls/GroupBox.xaml

<GroupBox Style="{DynamicResource GroupBoxStyle1}" x:Class="CustomControls.GroupBox"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
    <GroupBox.Resources>
        <BorderGapMaskConverter x:Key="BorderGapMaskConverter"/>
        <Style x:Key="GroupBoxStyle1" TargetType="{x:Type GroupBox}">
            <Setter Property="BorderBrush" Value="#DCDCDC"/>
            <Setter Property="BorderThickness" Value="1"/>
            <Setter Property="Template">
                <Setter.Value>
                    <ControlTemplate TargetType="{x:Type GroupBox}">
                        <Grid UseLayoutRounding="True">
                            <Grid.ColumnDefinitions>
                                <ColumnDefinition Width="6"/>
                                <ColumnDefinition Width="Auto"/>
                                <ColumnDefinition Width="*"/>
                                <ColumnDefinition Width="6"/>
                            </Grid.ColumnDefinitions>
                            <Grid.RowDefinitions>
                                <RowDefinition Height="Auto"/>
                                <RowDefinition Height="Auto"/>
                                <RowDefinition Height="*"/>
                                <RowDefinition Height="1"/>
                            </Grid.RowDefinitions>
                            <Border BorderBrush="{TemplateBinding BorderBrush}" BorderThickness="{TemplateBinding BorderThickness}" Grid.ColumnSpan="4" Grid.RowSpan="2" Grid.Row="1" Margin="0,-1,0,0" SnapsToDevicePixels="true">
                                <Border.OpacityMask>
                                    <MultiBinding ConverterParameter="7" Converter="{StaticResource BorderGapMaskConverter}">
                                        <Binding ElementName="Header" Path="ActualWidth"/>
                                        <Binding Path="ActualWidth" RelativeSource="{RelativeSource Self}"/>
                                        <Binding Path="ActualHeight" RelativeSource="{RelativeSource Self}"/>
                                    </MultiBinding>
                                </Border.OpacityMask>
                            </Border>
                            <Border x:Name="Header" Grid.Column="1" Padding="2,0,2,0" Grid.RowSpan="2" Grid.Row="0">
                                <ContentPresenter ContentSource="Header" RecognizesAccessKey="True" Margin="0,-1,0,0"/>
                            </Border>
                            <ContentPresenter Grid.Column="0" Grid.ColumnSpan="4" Margin="{TemplateBinding Padding}" Grid.Row="0" Grid.RowSpan="3"/>
                        </Grid>
                    </ControlTemplate>
                </Setter.Value>
            </Setter>
        </Style>
    </GroupBox.Resources>
</GroupBox>