fix: Don't underline HyperlinkButton if not ImplicitTextBlock

src/Uno.UI.RuntimeTests/Tests/Windows_UI_Xaml_Controls/HyperlinkButtonTests/Given_HyperlinkButton.cs

@@ -0,0 +1,33 @@
+using System;
+using System.Collections.Generic;
+using System.Text;
+using System.Threading.Tasks;
+using Microsoft.VisualStudio.TestTools.UnitTesting;
+using Private.Infrastructure;
+using Windows.UI.Text;
+using Windows.UI.Xaml.Controls;
+using Windows.UI.Xaml.Media;
+
+namespace Uno.UI.RuntimeTests.Tests.Windows_UI_Xaml_Controls.HyperlinkButtonTests
+{
+	[TestClass]
+	public class Given_HyperlinkButton
+	{
+		[TestMethod]
+		[RunsOnUIThread]
+		public async Task When_HyperlinkButton_With_Implicit_Content_Should_Be_UnderlinedAsync()
+		{
+			var SUT = new HyperlinkButtonPage();
+			TestServices.WindowHelper.WindowContent = SUT;
+			await TestServices.WindowHelper.WaitForIdle();
+
+			var underlinedImplicitTextBlock = VisualTreeHelper.GetChild(SUT.ShouldBeUnderlinedHyperlinkButton.GetTemplateChild("ContentPresenter"), 0) as TextBlock;
+			Assert.IsInstanceOfType(underlinedImplicitTextBlock, typeof(ImplicitTextBlock));
+			Assert.AreEqual(TextDecorations.Underline, underlinedImplicitTextBlock.TextDecorations);
+
+			var notUnderlinedTextBlock = VisualTreeHelper.GetChild(SUT.ShouldNotBeUnderlinedHyperlinkButton.GetTemplateChild("ContentPresenter"), 0) as TextBlock;
+			Assert.IsNotInstanceOfType(notUnderlinedTextBlock, typeof(ImplicitTextBlock));
+			Assert.AreEqual(TextDecorations.None, notUnderlinedTextBlock.TextDecorations);
+		}
+	}
+}

src/Uno.UI.RuntimeTests/Tests/Windows_UI_Xaml_Controls/HyperlinkButtonTests/HyperlinkButtonPage.xaml

@@ -0,0 +1,17 @@
+<Page
+    x:Class="Uno.UI.RuntimeTests.Tests.Windows_UI_Xaml_Controls.HyperlinkButtonTests.HyperlinkButtonPage"
+    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
+    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+    xmlns:local="using:Uno.UI.RuntimeTests.Tests.Windows_UI_Xaml_Controls.HyperlinkButtonTests"
+    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
+    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
+    mc:Ignorable="d"
+    Background="{ThemeResource ApplicationPageBackgroundThemeBrush}">
+
+    <StackPanel>
+		<HyperlinkButton x:Name="ShouldBeUnderlined_HyperlinkButton">Should be underlined text</HyperlinkButton>
+		<HyperlinkButton x:Name="ShouldNotBeUnderlined_HyperlinkButton">
+			<TextBlock x:Name="ShouldNotBeUnderlined_TextBlock">Shouldn't be underlined text</TextBlock>
+		</HyperlinkButton>
+	</StackPanel>
+</Page>

src/Uno.UI.RuntimeTests/Tests/Windows_UI_Xaml_Controls/HyperlinkButtonTests/HyperlinkButtonPage.xaml.cs

@@ -0,0 +1,34 @@
+using System;
+using System.Collections.Generic;
+using System.IO;
+using System.Linq;
+using System.Runtime.InteropServices.WindowsRuntime;
+using Windows.Foundation;
+using Windows.Foundation.Collections;
+using Windows.UI.Xaml;
+using Windows.UI.Xaml.Controls;
+using Windows.UI.Xaml.Controls.Primitives;
+using Windows.UI.Xaml.Data;
+using Windows.UI.Xaml.Input;
+using Windows.UI.Xaml.Media;
+using Windows.UI.Xaml.Navigation;
+
+// The Blank Page item template is documented at https://go.microsoft.com/fwlink/?LinkId=234238
+
+namespace Uno.UI.RuntimeTests.Tests.Windows_UI_Xaml_Controls.HyperlinkButtonTests
+{
+	/// <summary>
+	/// An empty page that can be used on its own or navigated to within a Frame.
+	/// </summary>
+	public sealed partial class HyperlinkButtonPage : Page
+	{
+		public HyperlinkButton ShouldBeUnderlinedHyperlinkButton => ShouldBeUnderlined_HyperlinkButton;
+		public HyperlinkButton ShouldNotBeUnderlinedHyperlinkButton => ShouldNotBeUnderlined_HyperlinkButton;
+		public TextBlock ShouldNotBeUnderlinedTextBlock => ShouldNotBeUnderlined_TextBlock;
+
+		public HyperlinkButtonPage()
+		{
+			this.InitializeComponent();
+		}
+	}
+}

src/Uno.UI.RuntimeTests/Uno.UI.RuntimeTests.csproj

@@ -112,6 +112,14 @@
 		</ProjectReference>
 	</ItemGroup>
 
+	<ItemGroup>
+	  <None Remove="Tests\Windows_UI_Xaml_Controls\HyperlinkButtonTests\HyperlinkButtonPage.xaml" />
+	</ItemGroup>
+
+	<ItemGroup>
+	  <UpToDateCheckInput Remove="Tests\Windows_UI_Xaml_Controls\HyperlinkButtonTests\HyperlinkButtonPage.xaml" />
+	</ItemGroup>
+
 	<ItemGroup>
 		<Compile Include="Tests\Windows_UI_Xaml_Controls\HtmlElementAttributeTests\Given_HtmlElementAttribute.Wasm.cs" />
 	</ItemGroup>

src/Uno.UI/UI/Xaml/Controls/Button/HyperlinkButton.cs

@@ -19,9 +19,26 @@ public HyperlinkButton()
 			DefaultStyleKey = typeof(HyperlinkButton);
 		}
 
-		/// <summary>
-		/// Gets or sets the Uniform Resource Identifier (URI) to navigate to when the HyperlinkButton is clicked.
-		/// </summary>
+		protected override void OnApplyTemplate()
+		{
+			base.OnApplyTemplate();
+
+			// This differs from UWP, where it looks for a template child named "ContentPresenter", 
+			// but ultimately sets the underline on the TextBlock of the first ContentPresenter with a {TemplateBinding Content}.
+			// UWP also doesn't seem to do this in OnApplyTemplate (it's done between the first Measure and the first Arrange).
+			if (GetTemplateChild("ContentPresenter") is ContentPresenter contentPresenter)
+			{
+				// Forces ContentPresenter to materialize its template.
+				contentPresenter.Measure(new Size(0, 0));
+				if (VisualTreeHelper.GetChildrenCount(contentPresenter) == 1 && VisualTreeHelper.GetChild(contentPresenter, 0) is ImplicitTextBlock textBlock)
+				{
+					textBlock.TextDecorations = Windows.UI.Text.TextDecorations.Underline;
+				}
+			}
+		}
+
+		#region NavigateUri
+
 		public Uri NavigateUri
 		{
 			get => (Uri)GetValue(NavigateUriProperty);