Avalonia/src/Avalonia.Themes.Simple/Controls/GroupBox.xaml

<ResourceDictionary xmlns="https://github.com/avaloniaui"
                    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
                    xmlns:converters="clr-namespace:Avalonia.Controls.Converters;assembly=Avalonia.Controls"
                    x:ClassModifier="internal">
  <Design.PreviewWith>
    <GroupBox Padding="10" Margin="10">
      <TextBlock Text="Hello World" />
    </GroupBox>
  </Design.PreviewWith>
  <converters:BorderGapMaskConverter x:Key="BorderGapMaskConverter" />

  <ControlTheme x:Key="{x:Type GroupBox}"
                TargetType="GroupBox">
    <Setter Property="BorderBrush" Value="{DynamicResource ThemeBorderMidBrush}"/>
    <Setter Property="BorderThickness" Value="1"/>
    <Setter Property="CornerRadius" Value="4"/>
    <Setter Property="Template">
      <ControlTemplate>
        <Grid x:Name="RootGrid" UseLayoutRounding="true" ColumnDefinitions="6,Auto,*,6" RowDefinitions="Auto,Auto,*,6">
          <!-- Separate Border for the background because if the background is set in the Border with the Header the opacity mask will be applied to the background as well. -->
          <Border Grid.Row="1"
                  Grid.RowSpan="3"
                  Grid.Column="0"
                  Grid.ColumnSpan="4"
                  CornerRadius="{TemplateBinding CornerRadius}"
                  BorderThickness="{TemplateBinding BorderThickness}"
                  BorderBrush="Transparent"
                  Background="{TemplateBinding Background}"/>

          <Border Grid.Row="1"
                  Grid.RowSpan="3"
                  Grid.ColumnSpan="4"
                  CornerRadius="{TemplateBinding CornerRadius}"
                  BorderThickness="{TemplateBinding BorderThickness}"
                  BorderBrush="{TemplateBinding BorderBrush}">
            <Border.OpacityMask>
              <MultiBinding Converter="{StaticResource BorderGapMaskConverter}"
                            ConverterParameter="6">
                <Binding ElementName="Header"
                         Path="Bounds.Width"/>
                <Binding RelativeSource="{RelativeSource Self}"
                         Path="Bounds.Width"/>
                <Binding RelativeSource="{RelativeSource Self}"
                         Path="Bounds.Height"/>
              </MultiBinding>
            </Border.OpacityMask>
          </Border>

          <!-- ContentPresenter for the header -->
          <Border x:Name="Header"
                  Padding="3,0,3,0"
                  Grid.Row="0"
                  Grid.RowSpan="2"
                  Grid.Column="1">
            <ContentPresenter Name="PART_HeaderPresenter"
                              Content="{TemplateBinding Header}"
                              ContentTemplate="{TemplateBinding HeaderTemplate}"
                              HorizontalContentAlignment="{TemplateBinding HorizontalContentAlignment}"
                              VerticalContentAlignment="{TemplateBinding VerticalContentAlignment}"
                              UseLayoutRounding="{TemplateBinding UseLayoutRounding}" />
          </Border>
          <!-- Primary content for GroupBox -->
          <ContentPresenter Grid.Row="2"
                            Grid.Column="1"
                            Grid.ColumnSpan="2"
                            Name="PART_ContentPresenter"
                            ContentTemplate="{TemplateBinding ContentTemplate}"
                            Content="{TemplateBinding Content}"
                            VerticalContentAlignment="{TemplateBinding VerticalContentAlignment}"
                            HorizontalContentAlignment="{TemplateBinding HorizontalContentAlignment}"
                            UseLayoutRounding="{TemplateBinding UseLayoutRounding}"
                            Margin="{TemplateBinding Padding}" />
        </Grid>
      </ControlTemplate>
    </Setter>

    <Style Selector="^ /template/ ContentPresenter#PART_ContentPresenter">
      <Setter Property="RecognizesAccessKey" Value="True" />
    </Style>

    <Style Selector="^ /template/ ContentPresenter#PART_HeaderPresenter">
      <Setter Property="RecognizesAccessKey" Value="True" />
    </Style>
</ControlTheme>
</ResourceDictionary>