Fix Markdown preview crash on UTF-8 files with >2MB size and <1.5M characters

src/modules/previewpane/MarkdownPreviewHandler/MarkdownPreviewHandlerControl.cs

@@ -182,8 +182,9 @@ public override void DoPreview<T>(T dataSource)
 
                         // WebView2.NavigateToString() limitation
                         // See https://learn.microsoft.com/dotnet/api/microsoft.web.webview2.core.corewebview2.navigatetostring?view=webview2-dotnet-1.0.864.35#remarks
-                        // While testing the limit, it turned out it is ~1.5MB, so to be on a safe side we go for 1.5m bytes
-                        if (markdownHTML.Length > 1_500_000)
+                        // While testing the limit, it turned out it is ~1.5MB of UTF-8 encoded content, so to be on the safe side we check the UTF-8 byte count.
+                        // Using character count (string.Length) is not sufficient because multi-byte UTF-8 characters (e.g. CJK) can cause the byte size to exceed the limit even when the character count is below it.
+                        if (System.Text.Encoding.UTF8.GetByteCount(markdownHTML) > 1_500_000)
                         {
                             string filename = _webView2UserDataFolder + "\\" + Guid.NewGuid().ToString() + ".html";
                             File.WriteAllText(filename, markdownHTML);

src/modules/previewpane/UnitTests-MarkdownPreviewHandler/MarkdownPreviewHandlerTest.cs

@@ -4,6 +4,9 @@
 
 using System;
 using System.Drawing;
+using System.IO;
+using System.Reflection;
+using System.Text;
 using System.Threading;
 using System.Windows.Forms;
 
@@ -16,6 +19,9 @@ namespace MarkdownPreviewHandlerUnitTests
     [STATestClass]
     public class MarkdownPreviewHandlerTest
     {
+        private const int NavigateToStringUtf8LimitInBytes = 1_500_000;
+        private const int OversizedUtf8FileThresholdInBytes = 2_000_000;
+
         // A long timeout is needed. WebView2 can take a long time to load the first time in some CI systems.
         private static readonly int HardTimeoutInMilliseconds = 60000;
         private static readonly int SleepTimeInMilliseconds = 200;
@@ -161,5 +167,141 @@ public void MarkdownPreviewHandlerControlUpdateInfobarSettingsWhenDoPreviewIsCal
                 Assert.AreEqual(true, ((RichTextBox)markdownPreviewHandlerControl.Controls[1]).Multiline);
             }
         }
+
+        [TestMethod]
+        public void MarkdownPreviewHandlerControlUsesTempFileNavigationWhenUtf8ByteCountExceedsThresholdWithMultiByteCharacters()
+        {
+            string content = new string('漢', 700_000);
+
+            Assert.IsTrue(content.Length < NavigateToStringUtf8LimitInBytes);
+            Assert.IsTrue(Encoding.UTF8.GetByteCount(content) > OversizedUtf8FileThresholdInBytes);
+
+            string filePath = CreateMarkdownFile(content);
+
+            try
+            {
+                using (var markdownPreviewHandlerControl = new MarkdownPreviewHandlerControl())
+                {
+                    markdownPreviewHandlerControl.DoPreview(filePath);
+
+                    WaitForBrowserControl(markdownPreviewHandlerControl);
+
+                    AssertUsesTempFileNavigation(markdownPreviewHandlerControl);
+                }
+            }
+            finally
+            {
+                DeleteFileIfExists(filePath);
+            }
+        }
+
+        [TestMethod]
+        public void MarkdownPreviewHandlerControlUsesNavigateToStringWhenContentIsWithinCharacterAndUtf8ByteThresholds()
+        {
+            string content = new string('a', 10_000);
+
+            Assert.IsTrue(content.Length < NavigateToStringUtf8LimitInBytes);
+            Assert.IsTrue(Encoding.UTF8.GetByteCount(content) < NavigateToStringUtf8LimitInBytes);
+
+            string filePath = CreateMarkdownFile(content);
+
+            try
+            {
+                using (var markdownPreviewHandlerControl = new MarkdownPreviewHandlerControl())
+                {
+                    markdownPreviewHandlerControl.DoPreview(filePath);
+
+                    WaitForBrowserControl(markdownPreviewHandlerControl);
+
+                    AssertUsesNavigateToString(markdownPreviewHandlerControl);
+                }
+            }
+            finally
+            {
+                DeleteFileIfExists(filePath);
+            }
+        }
+
+        [TestMethod]
+        public void MarkdownPreviewHandlerControlUsesTempFileNavigationWhenAsciiContentExceedsCharacterThreshold()
+        {
+            string content = new string('a', 1_600_000);
+
+            Assert.IsTrue(content.Length > NavigateToStringUtf8LimitInBytes);
+            Assert.IsTrue(Encoding.UTF8.GetByteCount(content) > NavigateToStringUtf8LimitInBytes);
+
+            string filePath = CreateMarkdownFile(content);
+
+            try
+            {
+                using (var markdownPreviewHandlerControl = new MarkdownPreviewHandlerControl())
+                {
+                    markdownPreviewHandlerControl.DoPreview(filePath);
+
+                    WaitForBrowserControl(markdownPreviewHandlerControl);
+
+                    AssertUsesTempFileNavigation(markdownPreviewHandlerControl);
+                }
+            }
+            finally
+            {
+                DeleteFileIfExists(filePath);
+            }
+        }
+
+        private static void WaitForBrowserControl(MarkdownPreviewHandlerControl markdownPreviewHandlerControl)
+        {
+            int beforeTick = Environment.TickCount;
+
+            while (markdownPreviewHandlerControl.Controls.Count == 0 && Environment.TickCount < beforeTick + HardTimeoutInMilliseconds)
+            {
+                Application.DoEvents();
+                Thread.Sleep(SleepTimeInMilliseconds);
+            }
+
+            Assert.AreEqual(1, markdownPreviewHandlerControl.Controls.Count);
+            Assert.IsInstanceOfType(markdownPreviewHandlerControl.Controls[0], typeof(WebView2));
+        }
+
+        private static void AssertUsesTempFileNavigation(MarkdownPreviewHandlerControl markdownPreviewHandlerControl)
+        {
+            Uri localFileUri = GetLocalFileUri(markdownPreviewHandlerControl);
+
+            Assert.IsNotNull(localFileUri);
+            Assert.IsTrue(File.Exists(localFileUri.LocalPath));
+            Assert.AreEqual(localFileUri, ((WebView2)markdownPreviewHandlerControl.Controls[0]).Source);
+        }
+
+        private static void AssertUsesNavigateToString(MarkdownPreviewHandlerControl markdownPreviewHandlerControl)
+        {
+            Assert.IsNull(GetLocalFileUri(markdownPreviewHandlerControl));
+        }
+
+        private static Uri GetLocalFileUri(MarkdownPreviewHandlerControl markdownPreviewHandlerControl)
+        {
+            FieldInfo localFileUriField = typeof(MarkdownPreviewHandlerControl).GetField("_localFileURI", BindingFlags.Instance | BindingFlags.NonPublic);
+
+            Assert.IsNotNull(localFileUriField);
+
+            return (Uri)localFileUriField.GetValue(markdownPreviewHandlerControl);
+        }
+
+        private static string CreateMarkdownFile(string content)
+        {
+            string generatedFilesDirectory = Path.Combine(AppContext.BaseDirectory, "HelperFiles", "Generated");
+            Directory.CreateDirectory(generatedFilesDirectory);
+
+            string filePath = Path.Combine(generatedFilesDirectory, $"{Guid.NewGuid():N}.md");
+            File.WriteAllText(filePath, content, Encoding.UTF8);
+            return filePath;
+        }
+
+        private static void DeleteFileIfExists(string filePath)
+        {
+            if (File.Exists(filePath))
+            {
+                File.Delete(filePath);
+            }
+        }
     }
 }