[Windows] Remove 2nd WebView used to add base tag when using HtmlWebViewSource (#21892)

### Description of Change

This PR removes the use of a 2nd "hidden" WebView2 that was used to
parse and add a HTML `base` tag to the `head` tag when setting the HTML
source of a WebView to a string.

This was done by appending the `base` tag script to the start of the
user's HTML string, which the WebView then adds into the `head` element.
While this is technically not valid HTML, all current browsers correct
this behavior.

This is a work-around for the lack of being able to set the base URL
when navigating to a string using WebView2
(MicrosoftEdge/WebView2Feedback#530).

As a bonus, using `HtmlWebViewSource` should now be 2x faster 😅

### Issues Fixed

Fixes #21631

Author: PureWeen

src/Controls/samples/Controls.Sample.UITests/Issues/Issue21631.xaml

@@ -0,0 +1,13 @@
+<?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.Issue21631">
+    <WebView
+        AutomationId="WaitForWebView"
+        x:Name="WaitForWebView">
+        <WebView.Source>
+            <HtmlWebViewSource 
+                Html="&lt;html&gt;&lt;body&gt;&lt;h1&gt;hello world&lt;/h1&gt;&lt;img src='appiconLargeTile.scale-100.png'/&gt;&lt;/html&gt;"/>
+        </WebView.Source>
+    </WebView>
+</ContentPage>

src/Controls/samples/Controls.Sample.UITests/Issues/Issue21631.xaml.cs

@@ -0,0 +1,15 @@
+using Microsoft.Maui.Controls;
+using Microsoft.Maui.Controls.Xaml;
+
+namespace Maui.Controls.Sample.Issues
+{
+	[XamlCompilation(XamlCompilationOptions.Compile)]
+	[Issue(IssueTracker.Github, 21631, "Injecting base tag in Webview2 works", PlatformAffected.UWP)]
+	public partial class Issue21631 : ContentPage
+	{
+		public Issue21631()
+		{
+			InitializeComponent();
+		}
+	}
+}

src/Controls/tests/UITests/Tests/Issues/Issue21631.cs

@@ -0,0 +1,24 @@
+using NUnit.Framework;
+using UITest.Appium;
+using UITest.Core;
+
+namespace Microsoft.Maui.AppiumTests.Issues
+{
+	public class Issue21631 : _IssuesUITest
+	{
+		public Issue21631(TestDevice device) : base(device) { }
+
+		public override string Issue => 
+			"Injecting base tag in Webview2 works";
+
+		[Test]
+		public async Task NavigateToStringWithWebviewWorks()
+		{
+			this.IgnoreIfPlatforms(new TestDevice[] { TestDevice.Android, TestDevice.Mac, TestDevice.iOS });
+
+			App.WaitForElement("WaitForWebView");
+			await Task.Delay(500);
+			VerifyScreenshot();
+		}
+	}
+}

src/Controls/tests/UITests/snapshots/windows/NavigateToStringWithWebviewWorks.png

@@ -1,7 +1,6 @@
 using System;
 using System.Diagnostics;
 using System.Text;
-using System.Text.RegularExpressions;
 using Microsoft.Maui.ApplicationModel;
 using Microsoft.UI.Xaml.Controls;
 using Windows.ApplicationModel;
@@ -28,8 +27,6 @@ public MauiWebView(WebViewHandler handler)
 			SetupPlatformEvents();
 		}
 
-		WebView2? _internalWebView;
-
 		// Arbitrary local host name for virtual folder mapping
 		const string LocalHostName = "appdir";
 		const string LocalScheme = $"https://{LocalHostName}/";
@@ -48,66 +45,29 @@ public MauiWebView(WebViewHandler handler)
 			: AppContext.BaseDirectory;
 
 		public async void LoadHtml(string? html, string? baseUrl)
-		{
+		{       
 			var mapBaseDirectory = false;
-
 			if (string.IsNullOrEmpty(baseUrl))
 			{
 				baseUrl = LocalScheme;
 				mapBaseDirectory = true;
 			}
 
-			// Generate a base tag for the document
-			var baseTag = $"<base href=\"{baseUrl}\"></base>";
-
-			string htmlWithBaseTag;
-
-			// Set up an internal WebView we can use to load and parse the original HTML string
-			// Make _internalWebView a field instead of local variable to avoid garbage collection
-			_internalWebView = new WebView2();
-
-			// TODO: For now, the CoreWebView2 won't be created without either setting Source or
-			// calling EnsureCoreWebView2Async().
-			await _internalWebView.EnsureCoreWebView2Async();
+			await EnsureCoreWebView2Async();
 
-			// When the 'navigation' to the original HTML string is done, we can modify it to include our <base> tag
-			_internalWebView.NavigationCompleted += async (sender, args) =>
-			{
-				// Generate a version of the <base> script with the correct <base> tag
-				var script = BaseInsertionScript.Replace("baseTag", baseTag, StringComparison.Ordinal);
-
-				// Run it and retrieve the updated HTML from our WebView
-				await sender.ExecuteScriptAsync(script);
-				htmlWithBaseTag = await sender.ExecuteScriptAsync("document.documentElement.outerHTML;");
+			if (mapBaseDirectory)
+		    {
+		    	CoreWebView2.SetVirtualHostNameToFolderMapping(
+		    		LocalHostName,
+		    		ApplicationPath,
+		    		Web.WebView2.Core.CoreWebView2HostResourceAccessKind.Allow);
+		    }
 
-				htmlWithBaseTag = Regex.Unescape(htmlWithBaseTag);
-				htmlWithBaseTag = htmlWithBaseTag.Remove(0, 1);
-				htmlWithBaseTag = htmlWithBaseTag.Remove(htmlWithBaseTag.Length - 1, 1);
-
-				await EnsureCoreWebView2Async();
+			// Insert script to set the base tag
+			var script = GetBaseTagInsertionScript(baseUrl);
+			var htmlWithScript = $"{script}\n{html}";
 
-				if (mapBaseDirectory)
-				{
-					CoreWebView2.SetVirtualHostNameToFolderMapping(
-						LocalHostName,
-						ApplicationPath,
-						Web.WebView2.Core.CoreWebView2HostResourceAccessKind.Allow);
-				}
-
-				// Set the HTML for the 'real' WebView to the updated HTML
-				NavigateToString(!string.IsNullOrEmpty(htmlWithBaseTag) ? htmlWithBaseTag : html);
-
-				// Free up memory after we're done with _internalWebView
-				if (_internalWebView.IsValid())
-				{
-					_internalWebView.Close();
-					_internalWebView = null;
-				}
-			};
-
-			// Kick off the initial navigation
-			if (_internalWebView.IsValid())
-				_internalWebView.NavigateToString(html);
+			NavigateToString(htmlWithScript);
 		}
 
 		public async void LoadUrl(string? url)
@@ -184,9 +144,16 @@ static bool IsWebView2DataUriWithBaseUrl(string? uri)
 				Convert.FromBase64String(
 					uri.Substring(dataUriBase64.Length)));
 
+			var localSchemeScript = GetBaseTagInsertionScript(LocalScheme);
 			return decodedHtml.Contains(
-				$"<base href=\"{LocalScheme}",
+				localSchemeScript,
 				StringComparison.OrdinalIgnoreCase);
 		}
+
+		static string GetBaseTagInsertionScript(string baseUrl)
+		{
+			var baseTag = $"<base href=\"{baseUrl}\"></base>";
+			return $"<script>{BaseInsertionScript.Replace("baseTag", baseTag, StringComparison.Ordinal)}</script>";
+		}
 	}
 }