Fix wrappanel new row issue

components/Primitives/src/WrapPanel/WrapPanel.cs

@@ -214,7 +214,7 @@ void Arrange(UIElement child, bool isLast = false)
             }
 
             var desiredMeasure = new UvMeasure(Orientation, child.DesiredSize);
-            if ((desiredMeasure.U + position.U + paddingEnd.U) > parentMeasure.U)
+            if ((desiredMeasure.U + position.U + paddingEnd.U) > parentMeasure.U || position.U >= parentMeasure.U)
             {
                 // next row!
                 position.U = paddingStart.U;

components/Primitives/tests/Primitives.Tests.projitems

@@ -50,6 +50,9 @@
     <Compile Include="$(MSBuildThisFileDirectory)WrapPanel\VerticalWrapPanelInsideParentWithLimitedHeight.xaml.cs">
       <DependentUpon>VerticalWrapPanelInsideParentWithLimitedHeight.xaml</DependentUpon>
     </Compile>
+    <Compile Include="$(MSBuildThisFileDirectory)WrapPanel\WrapPanelStretchLastChildInNewRow.xaml.cs">
+      <DependentUpon>WrapPanelStretchLastChildInNewRow.xaml</DependentUpon>
+    </Compile>
   </ItemGroup>
   <ItemGroup>
     <Page Include="$(MSBuildThisFileDirectory)DockPanel\DockPanelSample.xaml">
@@ -76,6 +79,10 @@
       <SubType>Designer</SubType>
       <Generator>MSBuild:Compile</Generator>
     </Page>
+    <Page Include="$(MSBuildThisFileDirectory)WrapPanel\WrapPanelStretchLastChildInNewRow.xaml">
+      <SubType>Designer</SubType>
+      <Generator>MSBuild:Compile</Generator>
+    </Page>
   </ItemGroup>
   <ItemGroup>
     <Page Include="$(MSBuildThisFileDirectory)SwitchPresenter\SwitchConverterBrushSample.xaml">

components/Primitives/tests/Test_WrapPanel_StretchChild.cs

@@ -101,4 +101,30 @@ public void HorizontalWrapPanelInsideParentWithLimitedWidthTest(HorizontalWrapPa
         double lastChildExpectedWidth = wrapPanel.ActualWidth - precedingChildren.Sum(child => child.ActualWidth);
         Assert.AreEqual(lastChildExpectedWidth, lastChild.ActualWidth, "Last child width not as expected.");
     }
+
+    /// <summary>
+    /// When a WrapPanel's width is constrained such that the last child wraps to a new row, the last child with Stretch alignment should fill the entire width of the new row.
+    /// </summary>
+    /// <param name="page"></param>
+    [TestCategory("WrapPanel")]
+    [UIThreadTestMethod]
+    public void WrapPanelStretchLastChildInNewRowTest(WrapPanelStretchLastChildInNewRow page)
+    {
+        var wrapPanel = page.FindDescendant<WrapPanel>();
+        Assert.IsNotNull(wrapPanel, "Could not find WrapPanel.");
+        Assert.IsFalse(wrapPanel.StretchChild is not StretchChild.Last, "WrapPanel StretchChild property not set to Last.");
+        Assert.IsFalse(wrapPanel.Children.Count < 1, "No children to test.");
+
+        var precedingChildren = wrapPanel.Children.Cast<FrameworkElement>().Take(wrapPanel.Children.Count - 1);
+
+        foreach (var child in precedingChildren)
+        {
+            double expectedWidth = child.DesiredSize.Width;
+            Assert.AreEqual(expectedWidth, child.ActualWidth, "Child width not as expected.");
+        }
+
+        var lastChild = wrapPanel.Children.Cast<FrameworkElement>().Last();
+        double lastChildExpectedWidth = wrapPanel.ActualWidth;
+        Assert.AreEqual(lastChildExpectedWidth, lastChild.ActualWidth, "Last child width not as expected.");
+    }
 }

components/Primitives/tests/WrapPanel/WrapPanelStretchLastChildInNewRow.xaml

@@ -0,0 +1,28 @@
+<Page x:Class="PrimitivesTests.WrapPanelStretchLastChildInNewRow"
+      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.WrapPanel"
+      xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
+      Background="{ThemeResource ApplicationPageBackgroundThemeBrush}"
+      mc:Ignorable="d">
+
+    <Grid>
+        <controls:WrapPanel Width="132"
+                            StretchChild="Last">
+            <Rectangle Width="44"
+                       Height="44"
+                       Fill="Red" />
+            <Rectangle Width="44"
+                       Height="44"
+                       Fill="Blue" />
+            <Rectangle Width="44"
+                       Height="44"
+                       Fill="Green" />
+            <Rectangle Height="44"
+                       Fill="Orange" />
+        </controls:WrapPanel>
+    </Grid>
+
+</Page>

components/Primitives/tests/WrapPanel/WrapPanelStretchLastChildInNewRow.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 WrapPanelStretchLastChildInNewRow : Page
+{
+    public WrapPanelStretchLastChildInNewRow()
+    {
+        this.InitializeComponent();
+    }
+}