Fix CardExpander has inconsistent CornerRadius

src/Wpf.Ui/Controls/CardExpander/CardExpander.xaml

@@ -106,6 +106,7 @@
                 <ControlTemplate TargetType="{x:Type controls:CardExpander}">
                     <ControlTemplate.Resources>
                         <converters:AnimationFactorToValueConverter x:Key="AnimationFactorToValueConverter" />
+                        <converters:CornerRadiusSplitConverter x:Key="CornerRadiusSplitConverter" />
                     </ControlTemplate.Resources>
 
                     <Grid>
@@ -120,8 +121,13 @@
                             Grid.Row="0"
                             Background="{TemplateBinding Background}"
                             BorderBrush="{TemplateBinding BorderBrush}"
-                            BorderThickness="1"
-                            CornerRadius="{TemplateBinding CornerRadius}">
+                            BorderThickness="1">
+                            <Border.CornerRadius>
+                                <MultiBinding Converter="{StaticResource CornerRadiusSplitConverter}" ConverterParameter="Top">
+                                    <Binding RelativeSource="{RelativeSource TemplatedParent}" Path="CornerRadius"/>
+                                    <Binding RelativeSource="{RelativeSource TemplatedParent}" Path="IsExpanded"/>
+                                </MultiBinding>
+                            </Border.CornerRadius>
                             <ToggleButton
                                 x:Name="ExpanderToggleButton"
                                 Margin="0"
@@ -171,8 +177,13 @@
                                 Background="{DynamicResource CardBackground}"
                                 BorderBrush="{TemplateBinding BorderBrush}"
                                 BorderThickness="1,0,1,1"
-                                CornerRadius="0,0,4,4"
                                 Visibility="Collapsed">
+                                <Border.CornerRadius>
+                                    <MultiBinding Converter="{StaticResource CornerRadiusSplitConverter}" ConverterParameter="Bottom">
+                                        <Binding RelativeSource="{RelativeSource TemplatedParent}" Path="CornerRadius"/>
+                                        <Binding RelativeSource="{RelativeSource TemplatedParent}" Path="IsExpanded"/>
+                                    </MultiBinding>
+                                </Border.CornerRadius>
                                 <ContentPresenter
                                     x:Name="ContentPresenter"
                                     Margin="{TemplateBinding ContentPadding}"
@@ -195,7 +206,6 @@
                     <ControlTemplate.Triggers>
                         <Trigger Property="IsExpanded" Value="True">
                             <!--  TODO: Update  -->
-                            <Setter TargetName="ToggleButtonBorder" Property="CornerRadius" Value="4,4,0,0" />
                             <Trigger.EnterActions>
                                 <BeginStoryboard>
                                     <Storyboard>

src/Wpf.Ui/Converters/CornerRadiusSplitConverter.cs

@@ -0,0 +1,44 @@
+// This Source Code Form is subject to the terms of the MIT License.
+// If a copy of the MIT was not distributed with this file, You can obtain one at https://opensource.org/licenses/MIT.
+// Copyright (C) Leszek Pomianowski and WPF UI Contributors.
+// All Rights Reserved.
+
+using System.Windows.Data;
+
+namespace Wpf.Ui.Converters;
+
+public class CornerRadiusSplitConverter : IMultiValueConverter
+{
+    public object Convert(object[] values, Type targetType, object parameter, CultureInfo culture)
+    {
+        var original = new CornerRadius(0);
+        if (values.Length > 0 && values[0] is CornerRadius cornerRadius)
+        {
+            original = cornerRadius;
+        }
+
+        bool isExpanded = false;
+        if (values.Length > 1 && values[1] is bool isExpand)
+        {
+            isExpanded = isExpand;
+        }
+
+        var side = (parameter as string) ?? "Top";
+
+        if (string.Equals(side, "Top", StringComparison.OrdinalIgnoreCase))
+        {
+            return isExpanded
+                ? new CornerRadius(original.TopLeft, original.TopRight, 0, 0)
+                : original;
+        }
+        else
+        {
+            return isExpanded
+                ? new CornerRadius(0, 0, original.BottomRight, original.BottomLeft)
+                : new CornerRadius(0);
+        }
+    }
+
+    public object[] ConvertBack(object value, Type[] targetTypes, object parameter, CultureInfo culture)
+        => throw new NotSupportedException();
+}