diff --git a/src/Avalonia.Base/Rect.cs b/src/Avalonia.Base/Rect.cs
index 58a8c56c8b..9c901254a6 100644
--- a/src/Avalonia.Base/Rect.cs
+++ b/src/Avalonia.Base/Rect.cs
@@ -1,7 +1,6 @@
 using System;
 using System.Globalization;
 using System.Numerics;
-using Avalonia.Animation.Animators;
 using Avalonia.Utilities;
 
 namespace Avalonia
diff --git a/src/Avalonia.Controls/Converters/BorderGapMaskConverter.cs b/src/Avalonia.Controls/Converters/BorderGapMaskConverter.cs
new file mode 100644
index 0000000000..913b2f1534
--- /dev/null
+++ b/src/Avalonia.Controls/Converters/BorderGapMaskConverter.cs
@@ -0,0 +1,126 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using System;
+using System.Collections.Generic;
+using System.Globalization;
+using Avalonia.Controls.Shapes;
+using Avalonia.Data.Converters;
+using Avalonia.Media;
+
+namespace Avalonia.Controls.Converters
+{
+    // Ported from https://github.com/dotnet/wpf/blob/main/src/Microsoft.DotNet.Wpf/src/PresentationFramework/System/Windows/Controls/BorderGapMaskConverter.cs
+
+    /// <summary>
+    /// Converter that generates the visual brush for <see cref="GroupBox"/>
+    /// </summary>
+    public class BorderGapMaskConverter : IMultiValueConverter
+    {
+        /// <summary>
+        /// Convert a value.
+        /// </summary>
+        /// <param name="values">values as produced by source binding</param>
+        /// <param name="targetType">target type</param>
+        /// <param name="parameter">converter parameter</param>
+        /// <param name="culture">culture information</param>
+        /// <returns>
+        /// Converted value.
+        /// Visual Brush that is used as the opacity mask for the Border
+        /// in the style for GroupBox.
+        /// </returns>
+        public object? Convert(IList<object?> values, Type targetType, object? parameter, CultureInfo culture)
+        {
+            //
+            // Parameter Validation
+            //
+            if (parameter == null ||
+                values == null ||
+                values.Count != 3 ||
+                values[0] is not double ||
+                values[1] is not double ||
+                values[2] is not double)
+            {
+                return AvaloniaProperty.UnsetValue;
+            }
+
+            if (parameter is not double && parameter is not string)
+            {
+                return AvaloniaProperty.UnsetValue;
+            }
+
+            //
+            // Conversion
+            //
+            double? headerWidth = (double?)values[0];
+            double? borderWidth = (double?)values[1];
+            double? borderHeight = (double?)values[2];
+
+            // Doesn't make sense to have a Grid
+            // with 0 as width or height
+            if (borderWidth == 0
+                || borderHeight == 0)
+            {
+                return null;
+            }
+
+            // Width of the line to the left of the header
+            // to be used to set the width of the first column of the Grid
+            double lineWidth;
+            if (parameter is string)
+            {
+                lineWidth = Double.Parse(((string)parameter), NumberFormatInfo.InvariantInfo);
+            }
+            else
+            {
+                lineWidth = (double)parameter;
+            }
+
+            Grid grid = new Grid
+            {
+                Width = borderWidth ?? 0,
+                Height = borderHeight ?? 0
+            };
+
+            ColumnDefinition colDef1 = new ColumnDefinition();
+            ColumnDefinition colDef2 = new ColumnDefinition();
+            ColumnDefinition colDef3 = new ColumnDefinition();
+            colDef1.Width = new GridLength(lineWidth);
+            colDef2.Width = new GridLength(headerWidth ?? 0);
+            colDef3.Width = new GridLength(1, GridUnitType.Star);
+            grid.ColumnDefinitions.Add(colDef1);
+            grid.ColumnDefinitions.Add(colDef2);
+            grid.ColumnDefinitions.Add(colDef3);
+            RowDefinition rowDef1 = new RowDefinition();
+            RowDefinition rowDef2 = new RowDefinition();
+            rowDef1.Height = new GridLength((borderHeight ?? 0) / 2);
+            rowDef2.Height = new GridLength(1, GridUnitType.Star);
+            grid.RowDefinitions.Add(rowDef1);
+            grid.RowDefinitions.Add(rowDef2);
+
+            Rectangle rectColumn1 = new Rectangle();
+            Rectangle rectColumn2 = new Rectangle();
+            Rectangle rectColumn3 = new Rectangle();
+            rectColumn1.Fill = Brushes.Black;
+            rectColumn2.Fill = Brushes.Black;
+            rectColumn3.Fill = Brushes.Black;
+
+            Grid.SetRowSpan(rectColumn1, 2);
+            Grid.SetRow(rectColumn1, 0);
+            Grid.SetColumn(rectColumn1, 0);
+
+            Grid.SetRow(rectColumn2, 1);
+            Grid.SetColumn(rectColumn2, 1);
+
+            Grid.SetRowSpan(rectColumn3, 2);
+            Grid.SetRow(rectColumn3, 0);
+            Grid.SetColumn(rectColumn3, 2);
+
+            grid.Children.Add(rectColumn1);
+            grid.Children.Add(rectColumn2);
+            grid.Children.Add(rectColumn3);
+
+            return (new VisualBrush(grid));
+        }
+    }
+}
diff --git a/src/Avalonia.Themes.Fluent/Controls/GroupBox.xaml b/src/Avalonia.Themes.Fluent/Controls/GroupBox.xaml
index 4278b91e36..5b31fdb55c 100644
--- a/src/Avalonia.Themes.Fluent/Controls/GroupBox.xaml
+++ b/src/Avalonia.Themes.Fluent/Controls/GroupBox.xaml
@@ -1,11 +1,13 @@
 <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">
@@ -18,62 +20,60 @@
       <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.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>
+          <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}"
-                                  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}"
+          <!-- 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}"
-                              UseLayoutRounding="{TemplateBinding UseLayoutRounding}"
-                              Margin="{TemplateBinding Padding}" />
+                              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>
@@ -84,7 +84,7 @@
 
     <Style Selector="^ /template/ ContentPresenter#PART_HeaderPresenter">
       <Setter Property="RecognizesAccessKey" Value="True" />
-    </Style> 
+    </Style>
 
   </ControlTheme>
 </ResourceDictionary>
diff --git a/src/Avalonia.Themes.Simple/Controls/GroupBox.xaml b/src/Avalonia.Themes.Simple/Controls/GroupBox.xaml
index 526ba7cc77..826c3ec2fe 100644
--- a/src/Avalonia.Themes.Simple/Controls/GroupBox.xaml
+++ b/src/Avalonia.Themes.Simple/Controls/GroupBox.xaml
@@ -1,11 +1,13 @@
 <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">
@@ -16,60 +18,58 @@
       <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.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>
+          <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}"
+          <!-- 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}"
-                              UseLayoutRounding="{TemplateBinding UseLayoutRounding}"
-                              Margin="{TemplateBinding Padding}" />
+                              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>
@@ -80,6 +80,6 @@
 
     <Style Selector="^ /template/ ContentPresenter#PART_HeaderPresenter">
       <Setter Property="RecognizesAccessKey" Value="True" />
-    </Style> 
-  </ControlTheme>
+    </Style>
+</ControlTheme>
 </ResourceDictionary>