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

<ResourceDictionary xmlns="https://github.com/avaloniaui"
                    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
                    x:ClassModifier="internal">
  <Design.PreviewWith>
    <GroupBox Padding="10" Margin="10">
      <TextBlock Text="Hello World" />
    </GroupBox>
  </Design.PreviewWith>

  <ControlTheme x:Key="{x:Type GroupBox}"
                TargetType="GroupBox">
    <Setter Property="BorderThickness" Value="{DynamicResource GroupBoxBorderThickness}" />
    <Setter Property="Background" Value="{DynamicResource GroupBoxBackground}" />
    <Setter Property="BorderBrush" Value="{DynamicResource GroupBoxBorderBrush}" />
    <Setter Property="Padding" Value="{DynamicResource GroupBoxPadding}" />
    <Setter Property="CornerRadius" Value="{DynamicResource ControlCornerRadius}" />
    <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>
                <VisualBrush>
                  <VisualBrush.Visual>
                    <Canvas Background="Transparent">
                      <Rectangle Fill="Transparent"
                                Width="{Binding #Header.Bounds.Width}"
                                Height="{Binding #Header.Bounds.Height}"
                                Canvas.Left="{Binding #Header.Bounds.X}"
                                Canvas.Top="{Binding #Header.Bounds.Y}"/>
                    </Canvas>
                  </VisualBrush.Visual>
                </VisualBrush>
              </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}"
                                  FontSize="{DynamicResource GroupBoxHeaderFontSize}"
                                  Foreground="{DynamicResource GroupBoxHeaderForeground}"
                                  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>