Added a UiTest (dotnet#19159)

src/Controls/samples/Controls.Sample.UITests/Issues/Issue19159.xaml

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+<?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"
+            xmlns:local="clr-namespace:Maui.Controls.Sample.Issues"
+             x:Class="Maui.Controls.Sample.Issues.Issue19159">
+       <TabBar>
+           <Tab>
+             <ShellContent
+                 Title="Page 1"
+                 ContentTemplate="{x:DataTemplate local:Issue19159ContentPage}">
+             </ShellContent>
+             <ShellContent
+                 Title="Page 2"
+                 ContentTemplate="{x:DataTemplate local:Issue19159ContentPage}"/>
+            </Tab>
+        </TabBar>
+</Shell>

src/Controls/samples/Controls.Sample.UITests/Issues/Issue19159.xaml.cs

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+using Microsoft.Maui.Controls;
+using Microsoft.Maui.Controls.Xaml;
+
+namespace Maui.Controls.Sample.Issues
+{
+	[XamlCompilation(XamlCompilationOptions.Compile)]
+	[Issue(IssueTracker.Github, 19159, "Top tab bar on Shell hides content", PlatformAffected.iOS)]
+	public partial class Issue19159 : Shell
+	{
+		public Issue19159()
+		{
+			InitializeComponent();
+		}
+	}
+
+	public class Issue19159ContentPage : ContentPage
+	{
+		public Issue19159ContentPage()
+		{
+			Content = new StackLayout()
+			{
+				AutomationId = "page",
+				HorizontalOptions = LayoutOptions.Fill,
+				VerticalOptions = LayoutOptions.Fill,
+				BackgroundColor = new Microsoft.Maui.Graphics.Color(255, 0, 0),
+				Margin = new Microsoft.Maui.Thickness(10)
+			};
+		}
+	}
+}

src/Controls/tests/UITests/Tests/Issues/Issue19159.cs

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+using NUnit.Framework;
+using UITest.Appium;
+using UITest.Core;
+
+namespace Microsoft.Maui.AppiumTests.Issues
+{
+	public class Issue19159 : _IssuesUITest
+	{
+		public Issue19159(TestDevice device) : base(device)
+		{
+		}
+
+		public override string Issue => "Top tab bar on Shell hides content";
+
+		[Test]
+		public void ContentShouldNotBeOverlaidByTopBar()
+		{
+			this.IgnoreIfPlatforms(new TestDevice[] { TestDevice.Mac, TestDevice.Windows | TestDevice.Android });
+			_ = App.WaitForElement("page1");
+
+			// The content should not be overlaid by top bar
+			App.Screenshot();
+		}
+	}
+}