Fix CollectionView group header not stretching to full width on Windows

src/Controls/src/Core/Platform/Windows/CollectionView/ItemsViewStyles.xaml

@@ -203,14 +203,14 @@
         </Setter>
     </Style>
 
-    <!-- Custom version of the style which removes the horizontal rule below the header -->
+    <!-- Custom style: removes horizontal rule, stretches content to full width, and zeroes container padding so MAUI header templates control their own spacing -->
     <Style TargetType="ListViewHeaderItem">
         <Setter Property="FontFamily" Value="{ThemeResource ContentControlThemeFontFamily}" />
         <Setter Property="FontSize" Value="{ThemeResource ListViewHeaderItemThemeFontSize}" />
         <Setter Property="Background" Value="{ThemeResource ListViewHeaderItemBackground}" />
         <Setter Property="Margin" Value="0,0,0,4" />
-        <Setter Property="Padding" Value="12,8,12,0" />
-        <Setter Property="HorizontalContentAlignment" Value="Left" />
+        <Setter Property="Padding" Value="0,0,0,0" />
+        <Setter Property="HorizontalContentAlignment" Value="Stretch" />
         <Setter Property="VerticalContentAlignment" Value="Top" />
         <Setter Property="MinHeight" Value="{ThemeResource ListViewHeaderItemMinHeight}" />
         <Setter Property="UseSystemFocusVisuals" Value="True" />
@@ -231,14 +231,14 @@
         </Setter>
     </Style>
 
-    <!-- Custom version of the style which removes the horizontal rule below the header -->
+    <!-- Custom style: removes horizontal rule, stretches content to full width, and zeroes container padding so MAUI header templates control their own spacing -->
     <Style TargetType="GridViewHeaderItem">
         <Setter Property="FontFamily" Value="{ThemeResource ContentControlThemeFontFamily}" />
         <Setter Property="FontSize" Value="{ThemeResource GridViewHeaderItemThemeFontSize}" />
         <Setter Property="Background" Value="{ThemeResource GridViewHeaderItemBackground}" />
         <Setter Property="Margin" Value="0,0,0,4" />
-        <Setter Property="Padding" Value="12,8,12,0" />
-        <Setter Property="HorizontalContentAlignment" Value="Left" />
+        <Setter Property="Padding" Value="0,0,0,0" />
+        <Setter Property="HorizontalContentAlignment" Value="Stretch" />
         <Setter Property="VerticalContentAlignment" Value="Top" />
         <Setter Property="MinHeight" Value="{ThemeResource GridViewHeaderItemMinHeight}" />
         <Setter Property="UseSystemFocusVisuals" Value="True" />

src/Controls/tests/TestCases.HostApp/Issues/Issue23038.cs

@@ -0,0 +1,79 @@
+namespace Maui.Controls.Sample.Issues;
+
+[Issue(IssueTracker.Github, 23038, "[Windows] GroupHeaderTemplate width smaller than ItemTemplate", PlatformAffected.UWP)]
+public class Issue23038 : ContentPage
+{
+	public Issue23038()
+	{
+		var teams = new List<Issue23038Team>
+		{
+			new("TeamA", "Team A", new List<Issue23038Member>
+			{
+				new("Member 1"), new("Member 2"), new("Member 3"),
+			}),
+			new("TeamB", "Team B", new List<Issue23038Member>
+			{
+				new("Member 4"), new("Member 5"),
+			}),
+		};
+
+		var collectionView = new CollectionView
+		{
+			AutomationId = "GroupedCollectionView",
+			IsGrouped = true,
+			ItemsSource = teams,
+			GroupHeaderTemplate = new DataTemplate(() =>
+			{
+				var label = new Label
+				{
+					BackgroundColor = Colors.LightGreen,
+					FontAttributes = FontAttributes.Bold,
+					Padding = new Thickness(5),
+				};
+				label.SetBinding(Label.TextProperty, nameof(Issue23038Team.Name));
+				label.SetBinding(Label.AutomationIdProperty, nameof(Issue23038Team.Key), stringFormat: "Header{0}");
+				return label;
+			}),
+			GroupFooterTemplate = new DataTemplate(() =>
+			{
+				var label = new Label
+				{
+					BackgroundColor = Colors.Orange,
+					Padding = new Thickness(5),
+				};
+				label.SetBinding(Label.TextProperty, "Count", stringFormat: "Total: {0}");
+				label.SetBinding(Label.AutomationIdProperty, nameof(Issue23038Team.Key), stringFormat: "Footer{0}");
+				return label;
+			}),
+			ItemTemplate = new DataTemplate(() =>
+			{
+				var label = new Label
+				{
+					Padding = new Thickness(5, 2),
+				};
+				label.SetBinding(Label.TextProperty, nameof(Issue23038Member.Name));
+				return label;
+			}),
+		};
+
+		Content = collectionView;
+	}
+}
+
+public class Issue23038Team : List<Issue23038Member>
+{
+	public Issue23038Team(string key, string name, List<Issue23038Member> members) : base(members)
+	{
+		Key = key;
+		Name = name;
+	}
+
+	public string Key { get; set; }
+	public string Name { get; set; }
+}
+
+public class Issue23038Member
+{
+	public Issue23038Member(string name) => Name = name;
+	public string Name { get; set; }
+}

src/Controls/tests/TestCases.HostApp/Issues/Issue23038_GridLayout.cs

@@ -0,0 +1,62 @@
+namespace Maui.Controls.Sample.Issues;
+
+[Issue(IssueTracker.Github, 23038, "[Windows] GroupHeaderTemplate width smaller than ItemTemplate (GridLayout)", PlatformAffected.UWP, issueTestNumber: 1)]
+public class Issue23038_GridLayout : ContentPage
+{
+	public Issue23038_GridLayout()
+	{
+		var teams = new List<Issue23038Team>
+		{
+			new("TeamA", "Team A", new List<Issue23038Member>
+			{
+				new("Member 1"), new("Member 2"), new("Member 3"), new("Member 4"),
+			}),
+			new("TeamB", "Team B", new List<Issue23038Member>
+			{
+				new("Member 5"), new("Member 6"),
+			}),
+		};
+
+		var collectionView = new CollectionView
+		{
+			AutomationId = "GroupedGridCollectionView",
+			IsGrouped = true,
+			ItemsSource = teams,
+			ItemsLayout = new GridItemsLayout(2, ItemsLayoutOrientation.Vertical),
+			GroupHeaderTemplate = new DataTemplate(() =>
+			{
+				var label = new Label
+				{
+					BackgroundColor = Colors.LightBlue,
+					FontAttributes = FontAttributes.Bold,
+					Padding = new Thickness(5),
+				};
+				label.SetBinding(Label.TextProperty, nameof(Issue23038Team.Name));
+				label.SetBinding(Label.AutomationIdProperty, nameof(Issue23038Team.Key), stringFormat: "GridHeader{0}");
+				return label;
+			}),
+			GroupFooterTemplate = new DataTemplate(() =>
+			{
+				var label = new Label
+				{
+					BackgroundColor = Colors.LightCoral,
+					Padding = new Thickness(5),
+				};
+				label.SetBinding(Label.TextProperty, "Count", stringFormat: "Total: {0}");
+				label.SetBinding(Label.AutomationIdProperty, nameof(Issue23038Team.Key), stringFormat: "GridFooter{0}");
+				return label;
+			}),
+			ItemTemplate = new DataTemplate(() =>
+			{
+				var label = new Label
+				{
+					Padding = new Thickness(5, 2),
+				};
+				label.SetBinding(Label.TextProperty, nameof(Issue23038Member.Name));
+				return label;
+			}),
+		};
+
+		Content = collectionView;
+	}
+}

src/Controls/tests/TestCases.Shared.Tests/Tests/Issues/Issue23038.cs

@@ -0,0 +1,52 @@
+using NUnit.Framework;
+using UITest.Appium;
+using UITest.Core;
+
+namespace Microsoft.Maui.TestCases.Tests.Issues;
+
+public class Issue23038 : _IssuesUITest
+{
+	public override string Issue => "[Windows] GroupHeaderTemplate width smaller than ItemTemplate";
+
+	public Issue23038(TestDevice device) : base(device) { }
+
+	[Test]
+	[Category(UITestCategories.CollectionView)]
+	[FailsOnAndroidWhenRunningOnXamarinUITest("Windows-only test: validates WinUI CollectionView group header styles")]
+	[FailsOnIOSWhenRunningOnXamarinUITest("Windows-only test: validates WinUI CollectionView group header styles")]
+	[FailsOnMacWhenRunningOnXamarinUITest("Windows-only test: validates WinUI CollectionView group header styles")]
+	public void GroupHeaderShouldStretchToFullWidth()
+	{
+		var collectionViewRect = App.WaitForElement("GroupedCollectionView").GetRect();
+		var headerARect = App.WaitForElement("HeaderTeamA").GetRect();
+		var footerARect = App.WaitForElement("FooterTeamA").GetRect();
+		var headerBRect = App.WaitForElement("HeaderTeamB").GetRect();
+		var footerBRect = App.WaitForElement("FooterTeamB").GetRect();
+
+		// Primary assertion: header must stretch to full CollectionView width.
+		// Tolerance of 20px accounts for the possible scrollbar width on Windows.
+		Assert.That(headerARect.Width, Is.EqualTo(collectionViewRect.Width).Within(20),
+			"Group header (TeamA) should stretch to full CollectionView width");
+
+		// Footer (regular list item) should also stretch to CollectionView width.
+		Assert.That(footerARect.Width, Is.EqualTo(collectionViewRect.Width).Within(20),
+			"Group footer (TeamA) should stretch to full CollectionView width");
+
+		// Header must match footer width — both stretch to full width.
+		// Tolerance of 5px accounts for device-pixel rounding under DPI scaling.
+		Assert.That(headerARect.Width, Is.EqualTo(footerARect.Width).Within(5),
+			"Group header (TeamA) width should match group footer width");
+
+		// Second group must also stretch — protects against virtualization/container-recycling regressions.
+		Assert.That(headerBRect.Width, Is.EqualTo(collectionViewRect.Width).Within(20),
+			"Group header (TeamB) should stretch to full CollectionView width");
+		Assert.That(headerBRect.Width, Is.EqualTo(footerBRect.Width).Within(5),
+			"Group header (TeamB) width should match group footer width");
+
+		// Vertical spacing sanity: header sits within the CollectionView and above its footer.
+		Assert.That(headerARect.Y, Is.GreaterThanOrEqualTo(collectionViewRect.Y),
+			"Group header should render at or below the CollectionView top edge");
+		Assert.That(headerARect.Y, Is.LessThan(footerARect.Y),
+			"Group header should render above its group footer");
+	}
+}

src/Controls/tests/TestCases.Shared.Tests/Tests/Issues/Issue23038_GridLayout.cs

@@ -0,0 +1,40 @@
+using NUnit.Framework;
+using UITest.Appium;
+using UITest.Core;
+
+namespace Microsoft.Maui.TestCases.Tests.Issues;
+
+public class Issue23038_GridLayout : _IssuesUITest
+{
+	public override string Issue => "[Windows] GroupHeaderTemplate width smaller than ItemTemplate (GridLayout)";
+
+	public Issue23038_GridLayout(TestDevice device) : base(device) { }
+
+	[Test]
+	[Category(UITestCategories.CollectionView)]
+	[FailsOnAndroidWhenRunningOnXamarinUITest("Windows-only test: validates WinUI CollectionView group header styles")]
+	[FailsOnIOSWhenRunningOnXamarinUITest("Windows-only test: validates WinUI CollectionView group header styles")]
+	[FailsOnMacWhenRunningOnXamarinUITest("Windows-only test: validates WinUI CollectionView group header styles")]
+	public void GridGroupHeaderShouldStretchToFullWidth()
+	{
+		var collectionViewRect = App.WaitForElement("GroupedGridCollectionView").GetRect();
+		var headerARect = App.WaitForElement("GridHeaderTeamA").GetRect();
+		var headerBRect = App.WaitForElement("GridHeaderTeamB").GetRect();
+
+		// In a GridItemsLayout, group headers use GridViewHeaderItem and should span the full width.
+		// (Group footers are regular grid items that only span one column, so we compare header to CollectionView.)
+		// Tolerance of 20px accounts for the possible scrollbar width on Windows.
+		Assert.That(headerARect.Width, Is.EqualTo(collectionViewRect.Width).Within(20),
+			"Grid group header (TeamA) should stretch to full CollectionView width");
+
+		// Second group must also stretch — protects against virtualization/container-recycling regressions.
+		Assert.That(headerBRect.Width, Is.EqualTo(collectionViewRect.Width).Within(20),
+			"Grid group header (TeamB) should stretch to full CollectionView width");
+
+		// Vertical sanity: TeamA header sits within the CollectionView and above TeamB's header.
+		Assert.That(headerARect.Y, Is.GreaterThanOrEqualTo(collectionViewRect.Y),
+			"Grid group header should render at or below the CollectionView top edge");
+		Assert.That(headerARect.Y, Is.LessThan(headerBRect.Y),
+			"First group header should render above the second group header");
+	}
+}