Fix WrapPanel stretching behavior for last item

components/Primitives/src/WrapPanel/WrapPanel.cs

@@ -97,6 +97,12 @@ public Thickness Padding
     /// <summary>
     /// Gets or sets a value indicating how to arrange child items
     /// </summary>
+    /// <remarks>
+    /// When the available size provided to the panel is infinite (for example,
+    /// when placed in a container with Auto sizing), the last child will not be
+    /// stretched. Attempting to stretch in this scenario would cause the element
+    /// to expand to an infinite size and result in a runtime exception.
+    /// </remarks>
     public StretchChild StretchChild
     {
         get { return (StretchChild)GetValue(StretchChildProperty); }
@@ -219,7 +225,8 @@ void Arrange(UIElement child, bool isLast = false)
             }
 
             // Stretch the last item to fill the available space
-            if (isLast)
+            // if the parent measure is not infinite
+            if (isLast && !double.IsInfinity(parentMeasure.U))
             {
                 desiredMeasure.U = parentMeasure.U - position.U;
             }

components/Primitives/tests/Primitives.Tests.projitems

@@ -33,10 +33,14 @@
     <Compile Include="$(MSBuildThisFileDirectory)Test_UniformGrid_FreeSpots.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Test_UniformGrid_RowColDefinitions.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Test_WrapPanel_BasicLayout.cs" />
+    <Compile Include="$(MSBuildThisFileDirectory)Test_WrapPanel_Infinity.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Test_WrapPanel_Visibility.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)UniformGrid\AutoLayoutFixedElementZeroZeroSpecialPage.xaml.cs">
       <DependentUpon>AutoLayoutFixedElementZeroZeroSpecialPage.xaml</DependentUpon>
     </Compile>
+    <Compile Include="$(MSBuildThisFileDirectory)WrapPanel\WrapPanelSample.xaml.cs">
+      <DependentUpon>WrapPanelSample.xaml</DependentUpon>
+    </Compile>
   </ItemGroup>
   <ItemGroup>
     <Page Include="$(MSBuildThisFileDirectory)DockPanel\DockPanelSample.xaml">
@@ -47,6 +51,10 @@
       <SubType>Designer</SubType>
       <Generator>MSBuild:Compile</Generator>
     </Page>
+    <Page Include="$(MSBuildThisFileDirectory)WrapPanel\WrapPanelSample.xaml">
+      <SubType>Designer</SubType>
+      <Generator>MSBuild:Compile</Generator>
+    </Page>
   </ItemGroup>
   <ItemGroup>
     <Page Include="$(MSBuildThisFileDirectory)SwitchPresenter\SwitchConverterBrushSample.xaml">

components/Primitives/tests/Test_WrapPanel_Infinity.cs

@@ -0,0 +1,48 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+using CommunityToolkit.Tests;
+using CommunityToolkit.WinUI.Controls;
+
+namespace PrimitivesTests;
+
+[TestClass]
+public class Test_WrapPanel_Infinity : VisualUITestBase
+{
+    [TestCategory("WrapPanel")]
+    [TestMethod]
+    public async Task Test_WrapPanel_InfinityWidth_WithStretchChild_Last()
+    {
+        await App.DispatcherQueue.EnqueueAsync(async () =>
+        {
+            var treeRoot = XamlReader.Load(@"<Page
+    xmlns=""http://schemas.microsoft.com/winfx/2006/xaml/presentation""
+    xmlns:x=""http://schemas.microsoft.com/winfx/2006/xaml""
+    xmlns:controls=""using:CommunityToolkit.WinUI.Controls"">
+    <Grid>
+        <Grid.ColumnDefinitions>
+            <ColumnDefinition Width=""Auto"" />
+        </Grid.ColumnDefinitions>
+        <controls:WrapPanel
+            x:Name=""WrapPanel""
+            Grid.Column=""0""
+            StretchChild=""Last"">
+            <Border />
+            <Border />
+            <Border />
+        </controls:WrapPanel>
+    </Grid>
+</Page>") as FrameworkElement;
+
+            Assert.IsNotNull(treeRoot, "Could not load XAML tree.");
+
+            // Initialize Visual Tree
+            await LoadTestContentAsync(treeRoot);
+
+            var wrapPanel = treeRoot.FindChild("WrapPanel") as WrapPanel;
+            Assert.IsNotNull(wrapPanel, "Could not find WrapPanel in tree.");
+            Assert.IsTrue(wrapPanel.IsLoaded, "WrapPanel is not loaded.");
+        });
+    }
+}

components/Primitives/tests/WrapPanel/WrapPanelSample.xaml

@@ -0,0 +1,17 @@
+<Page x:Class="PrimitivesTests.WrapPanelSample"
+      xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
+      xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+      xmlns:controls="using:CommunityToolkit.WinUI.Controls"
+      xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
+      xmlns:local="using:PrimitivesTests"
+      xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
+      Background="{ThemeResource ApplicationPageBackgroundThemeBrush}"
+      mc:Ignorable="d">
+
+    <!--
+        This is required for dynamic XAML loading.
+        See https://github.com/CommunityToolkit/WindowsCommunityToolkit/issues/1961
+    -->
+    <controls:WrapPanel />
+
+</Page>

components/Primitives/tests/WrapPanel/WrapPanelSample.xaml.cs

@@ -0,0 +1,13 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+namespace PrimitivesTests;
+
+public sealed partial class WrapPanelSample : Page
+{
+    public WrapPanelSample()
+    {
+        this.InitializeComponent();
+    }
+}