IllegalArgumentException when changing number of tabbar pages - fix

src/Controls/src/Core/Platform/Android/BottomNavigationViewUtils.cs

@@ -94,16 +94,27 @@ internal static async void SetupMenu(
 				else
 				{
 					menuItem = menu.GetItem(i);
-					SetMenuItemTitle(menuItem, item.title);
-					loadTasks.Add(SetMenuItemIcon(menuItem, item.icon, mauiContext));
+					if (menuItem.ItemId != i)
+					{
+						menu.RemoveItem(menuItem.ItemId);
+						loadTasks.Add(SetupMenuItem(item, menu, i, currentIndex, bottomView, mauiContext, out menuItem));
+					}
+					else
+					{
+						SetMenuItemTitle(menuItem, item.title);
+						loadTasks.Add(SetMenuItemIcon(menuItem, item.icon, mauiContext));
+					}
 				}
 
 				menuItems.Add(menuItem);
 			}
 
-			if (showMore)
+			var menuSize = menu.Size();
+			if (showMore && menu.GetItem(menuSize - 1).ItemId != MoreTabId)
 			{
 				var moreString = context.Resources.GetText(Resource.String.overflow_tab_title);
+				if (menuSize == maxBottomItems)
+					menu.RemoveItem(menu.GetItem(menuSize - 1).ItemId);
 				var menuItem = menu.Add(0, MoreTabId, 0, moreString);
 				menuItems.Add(menuItem);
 

src/Controls/tests/TestCases.Shared.Tests/Tests/Issues/Issue18251.cs

@@ -0,0 +1,26 @@
+using NUnit.Framework;
+using UITest.Appium;
+using UITest.Core;
+
+namespace Microsoft.Maui.TestCases.Tests.Issues
+{
+	public class Issue18251 : _IssuesUITest
+	{
+		public Issue18251(TestDevice device) : base(device) { }
+
+		public override string Issue => "IllegalArgumentException when changing number of tabbar pages";
+
+		[Test]
+		public void NoExceptionShouldBeThrownAddingShellTabs()
+		{
+			App.WaitForElement("button");
+
+			for (int i = 0; i < 2; i++)
+			{
+				App.Click("button");
+			}
+
+			// The test passes if no crash is observed
+		}
+	}
+}

src/Controls/tests/TestCases/Issues/Issue18251.xaml

@@ -0,0 +1,30 @@
+<?xml version="1.0" encoding="utf-8" ?>
+<Shell
+    xmlns="http://schemas.microsoft.com/dotnet/2021/maui"
+    xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
+    x:Class="Maui.Controls.Sample.Issues.Issue18251"
+    xmlns:local="clr-namespace:Maui.Controls.Sample.Issues"
+    Title="FlyoutTapClose"
+    x:Name="shell">
+
+   <TabBar FlexLayout.AlignSelf="Auto">
+        <ShellContent Title="Page 1" ContentTemplate="{DataTemplate local:Issue18251Page}"  />
+        <Tab Title="Page 2" IsVisible="{Binding IsLoggedIn}" >
+            <ShellContent Title="Page 2-A" ContentTemplate="{DataTemplate local:Issue18251Page}" IsVisible="{Binding IsLoggedIn}" />
+            <ShellContent Title="Page 2-B" ContentTemplate="{DataTemplate local:Issue18251Page}" IsVisible="{Binding IsLoggedIn}" />
+        </Tab>
+        <ShellContent Title="LogIn/Out"  ContentTemplate="{DataTemplate local:Issue18251Page}" IsVisible="{Binding IsLoggedIn}" />
+        <ShellContent Title="LogIn/Out"  ContentTemplate="{DataTemplate local:Issue18251Page}" IsVisible="{Binding IsLoggedOut}"/>
+        <ShellContent Title="Page 5"  ContentTemplate="{DataTemplate local:Issue18251Page}" IsVisible="{Binding IsLoggedOut}" />
+        <ShellContent Title="Page 6"  ContentTemplate="{DataTemplate local:Issue18251Page}" IsVisible="{Binding IsLoggedOut}" />
+        <ShellContent Title="Page 7"  ContentTemplate="{DataTemplate local:Issue18251Page}" IsVisible="{Binding IsLoggedIn}" />
+        <ShellContent Title="Page 8"  ContentTemplate="{DataTemplate local:Issue18251Page}" IsVisible="{Binding IsLoggedIn}"  />
+        <ShellContent Title="Page 9"  ContentTemplate="{DataTemplate local:Issue18251Page}" IsVisible="{Binding IsLoggedIn}" />
+        <ShellContent Title="Page 10"  ContentTemplate="{DataTemplate local:Issue18251Page}" IsVisible="{Binding IsLoggedIn}" />
+        <ShellContent Title="Page 11"  ContentTemplate="{DataTemplate local:Issue18251Page}" IsVisible="{Binding IsLoggedIn}" />
+        <ShellContent Title="Page 12"  ContentTemplate="{DataTemplate local:Issue18251Page}" />
+        <ShellContent Title="Page 13"  ContentTemplate="{DataTemplate local:Issue18251Page}" />
+        <ShellContent Title="Page 14"  ContentTemplate="{DataTemplate local:Issue18251Page}" />
+        <ShellContent Title="Page 15"  ContentTemplate="{DataTemplate local:Issue18251Page}" IsVisible="{Binding IsLoggedIn}" />
+    </TabBar>
+</Shell>

src/Controls/tests/TestCases/Issues/Issue18251.xaml.cs

@@ -0,0 +1,68 @@
+using Microsoft.Maui.Controls;
+
+namespace Maui.Controls.Sample.Issues
+{
+	[Issue(IssueTracker.Github, 18251, "IllegalArgumentException when changing number of tabbar pages", PlatformAffected.Android)]
+	public partial class Issue18251 : Shell
+	{
+		internal static Issue18251ViewModel ViewModel;
+
+		public Issue18251()
+		{
+			InitializeComponent();
+			BindingContext = ViewModel = new();
+		}
+	}
+
+	public class Issue18251Page : ContentPage
+	{
+		public Issue18251Page()
+		{
+			Content =
+				new VerticalStackLayout() {
+					new Button()
+					{
+						AutomationId = "button",
+						Command = Issue18251.ViewModel.LoginCommand,
+						Text = "Click to change tabs"
+					}
+				};
+		}
+	}
+
+	public class Issue18251ViewModel : BindableObject
+	{
+		private bool _isLoggedIn;
+		public bool IsLoggedIn
+		{
+			get => _isLoggedIn;
+			set
+			{
+				_isLoggedIn = value;
+				OnPropertyChanged();
+			}
+		}
+
+		private bool _isLoggedOut = true;
+		public bool IsLoggedOut
+		{
+			get => _isLoggedOut;
+			set
+			{
+				_isLoggedOut = value;
+				OnPropertyChanged();
+			}
+		}
+
+		public Command LoginCommand { get; set; }
+
+		public Issue18251ViewModel()
+		{
+			LoginCommand = new Command(() =>
+			{
+				IsLoggedIn = !IsLoggedIn;
+				IsLoggedOut = !IsLoggedIn;
+			});
+		}
+	}
+}