[Windows] Avoid Carousel changes the current position when window is resized

src/Controls/samples/Controls.Sample.UITests/Issues/Issue22000.xaml

@@ -0,0 +1,42 @@
+<?xml version="1.0" encoding="utf-8" ?>
+<ContentPage  
+  xmlns="http://schemas.microsoft.com/dotnet/2021/maui"
+  xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
+  x:Class="Maui.Controls.Sample.Issues.Issue22000">
+  <Grid
+      RowDefinitions="Auto, *, Auto">
+    <Grid.Resources>
+
+      <DataTemplate x:Key="SampleItemTemplate">
+        <Grid 
+          RowDefinitions="Auto, Auto"
+          BackgroundColor="{Binding Color, Mode=OneWay}">
+          <Label 
+            TextColor="White"
+            Text="{Binding Title, Mode=OneWay}" 
+            FontAttributes="Bold" />
+          <Label 
+            Grid.Row="1"
+            TextColor="White"
+            Text="{Binding Description, Mode=OneWay}" />
+        </Grid>
+      </DataTemplate>
+
+    </Grid.Resources>
+    <Label 
+      AutomationId="WaitForStubControl"
+      Text="Tap the Button several times to resize the CarouselView. If the CurrentItem not change, the test has passed."/>
+    <CarouselView 
+      x:Name="TestCarouselView"
+      Grid.Row="1"
+      ItemTemplate="{StaticResource SampleItemTemplate}"
+      Loop="False"
+      HeightRequest="250"
+      HorizontalOptions="Center"/>
+    <Button
+      AutomationId="UpdateSizeButton"
+      Grid.Row="2"
+      Text="Update Size"
+      Clicked="OnUpdateSizeClicked"/>
+  </Grid>
+</ContentPage>

src/Controls/samples/Controls.Sample.UITests/Issues/Issue22000.xaml.cs

@@ -0,0 +1,62 @@
+using System;
+using System.Collections.Generic;
+using Microsoft.Maui.Controls;
+using Microsoft.Maui.Graphics;
+
+namespace Maui.Controls.Sample.Issues
+{
+	[Issue(IssueTracker.Github, 22000, "[Windows] Carousel changes the current position when window is resized", PlatformAffected.UWP)]
+	public partial class Issue22000 : ContentPage
+	{
+		readonly Random _random;
+
+		public Issue22000()
+		{
+			InitializeComponent();
+
+			_random = new Random();
+
+			var exampleItems = new List<Issue22000Model>
+			{
+				new Issue22000Model( "First", "First CarouselView item", Colors.Red ),
+				new Issue22000Model( "Second", "Second CarouselView item", Colors.LightBlue ),
+				new Issue22000Model( "Third", "Third CarouselView item", Colors.Pink ),
+				new Issue22000Model( "Fourth", "Fourth CarouselView item", Colors.GreenYellow ),
+				new Issue22000Model( "Fifth", "Fifth CarouselView item", Colors.Purple ),
+			};
+
+			TestCarouselView.ItemsSource = exampleItems;
+
+			UpdateCarouselViewSize();
+		}
+
+		void OnUpdateSizeClicked(object sender, EventArgs e)
+		{
+			UpdateCarouselViewSize();
+		}
+
+		void UpdateCarouselViewSize()
+		{
+			var currentWidth = TestCarouselView.WidthRequest;
+
+			if (currentWidth == 400)
+				TestCarouselView.WidthRequest = 800;
+			else
+				TestCarouselView.WidthRequest = 400;
+		}
+	}
+
+	class Issue22000Model
+	{
+		public Issue22000Model(string title, string description, Color color)
+		{
+			Title = title;
+			Description = description;
+			Color = color;
+		}
+
+		public string Title { get; set; }
+		public string Description { get; set; }
+		public Color Color { get; set; }
+	}
+}

src/Controls/src/Core/Handlers/Items/CarouselViewHandler.Windows.cs

@@ -22,7 +22,8 @@ public partial class CarouselViewHandler : ItemsViewHandler<CarouselView>
 		ScrollViewer _scrollViewer;
 		WScrollBarVisibility? _horizontalScrollBarVisibilityWithoutLoop;
 		WScrollBarVisibility? _verticalScrollBarVisibilityWithoutLoop;
-		Size _currentSize;
+		Size _currentSize; 
+		bool _isCarouselViewReady;
 		NotifyCollectionChangedEventHandler _collectionChanged;
 		readonly WeakNotifyCollectionChangedProxy _proxy = new();
 
@@ -548,10 +549,31 @@ void Resize(Size newSize)
 			{
 				_currentSize = newSize;
 
-				UpdateItemsSource();
-				UpdateSnapPointsType();
-				UpdateSnapPointsAlignment();
-				UpdateCarouselViewInitialPosition();
+				if (_isCarouselViewReady)
+					InvalidateItemSize();
+				else
+					InitialSetup();
+
+				_isCarouselViewReady = true;
+			}
+		}
+
+		void InitialSetup()
+		{
+			UpdateItemsSource();
+			UpdateSnapPointsType();
+			UpdateSnapPointsAlignment();
+			UpdateCarouselViewInitialPosition();
+		}
+
+		void InvalidateItemSize()
+		{
+			foreach (var item in ListViewBase.GetChildren<ItemContentControl>())
+			{
+				item.ItemHeight = GetItemHeight();
+				item.ItemWidth = GetItemWidth();
+
+				item.InvalidateMeasure();
 			}
 		}
 	}

src/Controls/tests/UITests/Tests/Issues/Issue22000.cs

@@ -0,0 +1,29 @@
+using NUnit.Framework;
+using UITest.Appium;
+using UITest.Core;
+
+namespace Microsoft.Maui.AppiumTests.Issues
+{
+	public class Issue22000 : _IssuesUITest
+	{
+		public Issue22000(TestDevice device) : base(device) { }
+
+		public override string Issue => "[Windows] Carousel changes the current position when window is resized";
+
+		[Test]
+		[Category(UITestCategories.CarouselView)]
+		public void ResizeCarouselViewKeepsIndex()
+		{
+			this.IgnoreIfPlatforms(new TestDevice[] { TestDevice.Android, TestDevice.iOS, TestDevice.Mac });
+
+			App.WaitForElement("WaitForStubControl");
+
+			for (int i = 0; i < 10; i++)
+			{
+				App.Click("UpdateSizeButton");
+			}
+
+			VerifyScreenshot();
+		}
+	}
+}