Don’t call NSAttributedString with HTML from a background thread

src/Controls/src/Core/Label/Label.iOS.cs

@@ -36,19 +36,17 @@ public static void MapMaxLines(ILabelHandler handler, Label label)
 			handler.PlatformView?.UpdateMaxLines(label);
 		}
 
-		static void MapFormatting(ILabelHandler handler, Label label)
+		internal static void MapFormatting(ILabelHandler handler, Label label)
 		{
-			// we need to re-apply color and font for HTML labels
-			if (!label.HasFormattedTextSpans && label.TextType == TextType.Html)
+			if (IsPlainText(label))
+			{
+				LabelHandler.MapFormatting(handler, label);
+			}
+			else
 			{
 				handler.UpdateValue(nameof(ILabel.TextColor));
 				handler.UpdateValue(nameof(ILabel.Font));
 			}
-
-			if (!IsPlainText(label))
-				return;
-
-			LabelHandler.MapFormatting(handler, label);
 		}
 
 		void RecalculateSpanPositions(Size size)

src/Controls/src/Core/Platform/iOS/Extensions/LabelExtensions.cs

@@ -14,7 +14,22 @@ public static void UpdateText(this UILabel platformLabel, Label label)
 			switch (label.TextType)
 			{
 				case TextType.Html:
-					platformLabel.UpdateTextHtml(label);
+					// NOTE: Setting HTML text this will crash with some sort of consistency error.
+					// https://github.com/dotnet/maui/issues/25946
+					// Here we have to dispatch back the the main queue to avoid the crash.
+					// This is observed with CarouselView 1 but not with 2, so hopefully this
+					// will be just disappear once we switch.
+					CoreFoundation.DispatchQueue.MainQueue.DispatchAsync(() =>
+					{
+						platformLabel.UpdateTextHtml(label);
+
+						if (label.Handler is LabelHandler labelHandler)
+							Label.MapFormatting(labelHandler, label);
+
+						// NOTE: Because we are updating text outside the normal layout
+						// pass, we need to invalidate the measure for the next pass.
+						label.InvalidateMeasure();
+					});
 					break;
 
 				default:

src/Controls/tests/TestCases.HostApp/Issues/Issue25946.xaml

@@ -0,0 +1,34 @@
+<?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.Issue25946">
+    <Grid RowDefinitions="*,Auto">
+        <CarouselView x:Name="collectionView">
+            <CarouselView.ItemsSource>
+                <x:Array Type="{x:Type x:String}">
+                    <x:String>Item1</x:String>
+                    <x:String>Item2</x:String>
+                    <x:String>Item3</x:String>
+                </x:Array>
+            </CarouselView.ItemsSource>
+            <CarouselView.ItemTemplate>
+                <DataTemplate>
+                    <ContentView>
+                        <VerticalStackLayout HorizontalOptions="Center"
+                                             WidthRequest="200"
+                                             HeightRequest="200"
+                                             Background="Green">
+                            <Label Text="{Binding .}"/>
+                            <Label Text="{Binding .}"
+                                   TextType="Html"/>
+                        </VerticalStackLayout>
+                    </ContentView>
+                </DataTemplate>
+            </CarouselView.ItemTemplate>
+        </CarouselView>
+        <Button Grid.Row="1"
+                AutomationId="btnScrollToLastItem"
+                Clicked="ButtonClicked"
+                Text="Scroll to last Item"/>
+    </Grid>
+</ContentPage>

src/Controls/tests/TestCases.HostApp/Issues/Issue25946.xaml.cs

@@ -0,0 +1,16 @@
+namespace Maui.Controls.Sample.Issues
+{
+	[Issue(IssueTracker.Github, 25946, "App crashes on iOS 18 when placing html label in carousel view with > 2 elements", PlatformAffected.iOS)]
+	public partial class Issue25946 : ContentPage
+	{
+		public Issue25946()
+		{
+			InitializeComponent();
+		}
+
+		void ButtonClicked(object sender, EventArgs e)
+		{
+			collectionView.ScrollTo(1, animate: true);
+		}
+	}
+}

src/Controls/tests/TestCases.Shared.Tests/Tests/Issues/Issue25946.cs

@@ -0,0 +1,26 @@
+using NUnit.Framework;
+using UITest.Appium;
+using UITest.Core;
+
+namespace Microsoft.Maui.TestCases.Tests.Issues
+{
+	public class Issue25946 : _IssuesUITest
+	{
+		public Issue25946(TestDevice testDevice) : base(testDevice)
+		{
+		}
+
+		public override string Issue => "App crashes on iOS 18 when placing html label in carousel view with > 2 elements";
+
+		[Test]
+		[Category(UITestCategories.CarouselView)]
+		[Category(UITestCategories.Label)]
+		public void AppShouldNotCrash()
+		{
+			App.WaitForElement("btnScrollToLastItem");
+			App.Click("btnScrollToLastItem");
+
+			App.WaitForElement("Item2");
+		}
+	}
+}

src/Core/src/Platform/iOS/LabelExtensions.cs

@@ -88,6 +88,14 @@ internal static void UpdateTextHtml(this UILabel platformLabel, ILabel label)
 			};
 
 			NSError nsError = new();
+
+			// NOTE: Sometimes this will crash with some sort of consistency error.
+			// https://github.com/dotnet/maui/issues/25946
+			// The caller should ensure that this extension is dispatched. We cannot
+			// do it here as we need to re-apply the formatting and we cannot call
+			// into Controls from Core.
+			// This is observed with CarouselView 1 but not with 2, so hopefully this
+			// will be just disappear once we switch.
 #pragma warning disable CS8601
 			platformLabel.AttributedText = new NSAttributedString(text, attr, ref nsError);
 #pragma warning restore CS8601