[Android] Fix CollectionView items not resizing after orientation change on Shell pages

src/Controls/src/Core/Handlers/Items/Android/Adapters/ItemsViewAdapter.cs

@@ -152,6 +152,14 @@ protected virtual void BindTemplatedItemViewHolder(TemplatedItemViewHolder templ
 			templatedItemViewHolder.Bind(context, ItemsView);
 		}
 
+		/// <summary>
+		/// Clears any cached item size used by the MeasureFirstItem sizing strategy.
+		/// Called when the RecyclerView's size changes (e.g., after an orientation change).
+		/// </summary>
+		internal virtual void ClearMeasureCache()
+		{
+		}
+
 		void UpdateUsingItemTemplate()
 		{
 			_usingItemTemplate = ItemsView.ItemTemplate != null;

src/Controls/src/Core/Handlers/Items/Android/Adapters/StructuredItemsViewAdapter.cs

@@ -169,5 +169,10 @@ void SetStaticSize(Size size)
 		{
 			_size = size;
 		}
+
+		internal override void ClearMeasureCache()
+		{
+			_size = null;
+		}
 	}
 }

src/Controls/src/Core/Handlers/Items/Android/ItemContentView.cs

@@ -83,6 +83,17 @@ internal void HandleItemSizingStrategy(Action<Size> reportMeasure, Size? size)
 			_pixelSize = size;
 		}
 
+		/// <summary>
+		/// Invalidates the cached size so the next measure pass will re-measure the content.
+		/// Called when the parent RecyclerView's size changes (e.g., after orientation change).
+		/// </summary>
+		internal void InvalidateCachedSize()
+		{
+			_pixelSize = null;
+			_previousPixelWidth = -1;
+			_previousPixelHeight = -1;
+		}
+
 		protected override void OnLayout(bool changed, int l, int t, int r, int b)
 		{
 			if (Content == null)

src/Controls/src/Core/Handlers/Items/Android/MauiRecyclerView.cs

@@ -574,6 +574,35 @@ protected override void OnLayout(bool changed, int l, int t, int r, int b)
 			_scrollHelper?.AdjustScroll();
 		}
 
+		protected override void OnSizeChanged(int w, int h, int oldw, int oldh)
+		{
+			base.OnSizeChanged(w, h, oldw, oldh);
+
+			// When the RecyclerView's size changes (e.g., after an orientation change while
+			// the CollectionView was not visible), we need to invalidate all visible item views
+			// to ensure they are re-measured with the new dimensions.
+			if (oldw > 0 && oldh > 0 && (Math.Abs(w - oldw) > 1 || Math.Abs(h - oldh) > 1))
+			{
+				InvalidateItemMeasures();
+			}
+		}
+
+		void InvalidateItemMeasures()
+		{
+			// Clear the adapter's static size cache (used by MeasureFirstItem strategy)
+			ItemsViewAdapter?.ClearMeasureCache();
+
+			// Force all visible children to invalidate their cached sizes and re-measure
+			for (int i = 0; i < ChildCount; i++)
+			{
+				if (GetChildAt(i) is ItemContentView itemContentView)
+				{
+					itemContentView.InvalidateCachedSize();
+					itemContentView.ForceLayout();
+				}
+			}
+		}
+
 		protected override void Dispose(bool disposing)
 		{
 			base.Dispose(disposing);

src/Controls/src/Core/PublicAPI/net-android/PublicAPI.Unshipped.txt

@@ -11,3 +11,4 @@ override Microsoft.Maui.Controls.Platform.Compatibility.ShellSectionRenderer.OnH
 ~override Microsoft.Maui.Controls.Handlers.Items.RecyclerViewScrollListener<TItemsView, TItemsViewSource>.OnScrollStateChanged(AndroidX.RecyclerView.Widget.RecyclerView recyclerView, int newState) -> void
 ~override Microsoft.Maui.Controls.Handlers.Items.SelectableItemsViewAdapter<TItemsView, TItemsSource>.IsSelectionEnabled(Android.Views.ViewGroup parent, int viewType) -> bool
 override Microsoft.Maui.Controls.TitleBar.OnBindingContextChanged() -> void
+override Microsoft.Maui.Controls.Handlers.Items.MauiRecyclerView<TItemsView, TAdapter, TItemsViewSource>.OnSizeChanged(int w, int h, int oldw, int oldh) -> void

src/Controls/tests/TestCases.HostApp/Issues/Issue19667.cs

@@ -0,0 +1,53 @@
+namespace Maui.Controls.Sample.Issues;
+
+[Issue(IssueTracker.Github, 19667, "CollectionView contents not sizing correctly after orientation change", PlatformAffected.Android)]
+public class Issue19667 : TestShell
+{
+	protected override void Init()
+	{
+		ContentPage page1 = new ContentPage
+		{
+			Content = new Label
+			{
+				Text = "Page 1",
+				AutomationId = "Page1Label",
+				HorizontalOptions = LayoutOptions.Center,
+				VerticalOptions = LayoutOptions.Center
+			}
+		};
+
+		var items = Enumerable.Range(0, 15).ToList();
+
+		CollectionView collectionView = new CollectionView
+		{
+			AutomationId = "CollectionView19667",
+			ItemsSource = items,
+			ItemTemplate = new DataTemplate(() =>
+			{
+				Label label = new Label
+				{
+					VerticalTextAlignment = TextAlignment.Center
+				};
+				label.SetBinding(Label.TextProperty, new Binding(".", stringFormat: "Item {0}"));
+				label.SetBinding(AutomationIdProperty, new Binding(".", stringFormat: "CvItem{0}"));
+
+				Grid grid = new Grid
+				{
+					BackgroundColor = Colors.Beige,
+					Padding = new Thickness(20),
+					Margin = new Thickness(5, 2)
+				};
+				grid.Add(label);
+				return grid;
+			})
+		};
+
+		ContentPage page2 = new ContentPage
+		{
+			Content = collectionView
+		};
+
+		AddFlyoutItem(page1, "Page1");
+		AddFlyoutItem(page2, "CollectionViewPage");
+	}
+}

src/Controls/tests/TestCases.Shared.Tests/Tests/Issues/Issue19667.cs

@@ -0,0 +1,49 @@
+#if ANDROID || IOS // The test fails on Windows and MacCatalyst because the SetOrientation method, which is intended to change the device orientation, is only supported on mobile platforms iOS and Android.
+using NUnit.Framework;
+using UITest.Appium;
+using UITest.Core;
+
+namespace Microsoft.Maui.TestCases.Tests.Issues;
+
+public class Issue19667 : _IssuesUITest
+{
+	public Issue19667(TestDevice device) : base(device)
+	{
+	}
+
+	public override string Issue => "CollectionView contents not sizing correctly after orientation change";
+
+	[Test]
+	[Category(UITestCategories.CollectionView)]
+	public void CollectionViewItemsSizeCorrectlyAfterOrientationChange()
+	{
+		App.TapShellFlyoutIcon();
+		App.Tap("CollectionViewPage");
+		App.WaitForElement("CollectionView19667");
+		App.WaitForElement("CvItem0");
+
+		var portraitWidth = App.WaitForElement("CvItem0").GetRect().Width;
+
+		App.TapShellFlyoutIcon();
+		App.Tap("Page1");
+		App.WaitForElement("Page1Label");
+
+		App.SetOrientationLandscape();
+
+		App.TapShellFlyoutIcon();
+		App.Tap("CollectionViewPage");
+		App.WaitForElement("CollectionView19667");
+		App.WaitForElement("CvItem0");
+
+		var landscapeWidth = App.WaitForElement("CvItem0").GetRect().Width;
+		Assert.That(landscapeWidth, Is.GreaterThan(portraitWidth),
+			"CollectionView items should resize to landscape width after orientation change.");
+	}
+
+	[TearDown]
+	public void TearDown()
+	{
+		App.SetOrientationPortrait();
+	}
+}
+#endif