fix(text): harden UnicodeText surrogate, indexing, and ICU bidi fallback

- Add defensive draw-time bounds checks for highlighter/run-break/word-boundary and line metric indexing.

- Sanitize malformed UTF-16 surrogate units to U+FFFD before shaping.

- Retry ICU ubidi_setPara without embedding levels when explicit levels fail, with bidi handle cleanup.

- Keep/update unit and Skia runtime regression coverage for malformed surrogate and highlighter-range scenarios.

Author: agneszitte

src/Uno.UI.RuntimeTests/Tests/Windows_UI_Xaml_Controls/Given_TextBlock.cs

@@ -739,6 +739,66 @@ public async Task When_FontFamily_Changed(string font)
 			Assert.AreNotEqual(originalSize, SUT.DesiredSize);
 		}
 
+		[TestMethod]
+		[PlatformCondition(ConditionMode.Include, RuntimeTestPlatforms.Skia)]
+		public async Task When_TextBlock_With_Unpaired_Surrogate_Renders()
+		{
+			if (!ApiInformation.IsTypePresent("Microsoft.UI.Xaml.Media.Imaging.RenderTargetBitmap, Uno.UI"))
+			{
+				Assert.Inconclusive();
+			}
+
+			var text = "LTR \uD800 RTL שלום";
+			var sut = new TextBlock
+			{
+				Text = text,
+				TextWrapping = TextWrapping.Wrap
+			};
+			sut.TextHighlighters.Add(new TextHighlighter
+			{
+				Ranges =
+				{
+					new TextRange { StartIndex = 0, Length = text.Length }
+				}
+			});
+
+			var root = new Border
+			{
+				Width = 240,
+				Height = 120,
+				Child = sut
+			};
+
+			await UITestHelper.Load(root);
+			await UITestHelper.ScreenShot(root);
+		}
+
+		[TestMethod]
+		[PlatformCondition(ConditionMode.Include, RuntimeTestPlatforms.Skia)]
+		public async Task When_TextBlock_With_Unpaired_Surrogate_And_No_Highlighter_Renders()
+		{
+			if (!ApiInformation.IsTypePresent("Microsoft.UI.Xaml.Media.Imaging.RenderTargetBitmap, Uno.UI"))
+			{
+				Assert.Inconclusive();
+			}
+
+			var sut = new TextBlock
+			{
+				Text = "LTR \uD800 RTL שלום",
+				TextWrapping = TextWrapping.Wrap
+			};
+
+			var root = new Border
+			{
+				Width = 240,
+				Height = 120,
+				Child = sut
+			};
+
+			await UITestHelper.Load(root);
+			await UITestHelper.ScreenShot(root);
+		}
+
 		[TestMethod]
 #if !__ANDROID__
 		[Ignore("Android-only test for AndroidAssets backward compatibility")]

src/Uno.UI.Tests/Windows_UI_XAML_Controls/TextBlockTests/Given_TextBlock.cs

@@ -7,8 +7,8 @@
 using Microsoft.UI.Xaml;
 using Microsoft.UI.Xaml.Documents;
 using Microsoft.UI.Xaml.Controls;
-using System.Drawing;
 using Microsoft.UI.Xaml.Media;
+using Windows.Foundation;
 
 namespace Uno.UI.Tests.Windows_UI_XAML_Controls.TextBlockTests
 {
@@ -51,6 +51,55 @@ public void When_Changing_Foreground_Property()
 		}
 #endif
 
+		[TestMethod]
+		public void When_Text_Has_Unpaired_Surrogate_Does_Not_Throw()
+		{
+			var tb = new TextBlock
+			{
+				Text = "A\uD800B\uDC00"
+			};
+
+			tb.Measure(new Size(300, 80));
+		}
+
+		[TestMethod]
+		public void When_Text_Has_Unpaired_Surrogate_With_Highlighter_Does_Not_Throw()
+		{
+			var tb = new TextBlock
+			{
+				Text = "A\uD800B\uDC00"
+			};
+
+			tb.TextHighlighters.Add(new TextHighlighter
+			{
+				Ranges =
+				{
+					new TextRange { StartIndex = 0, Length = tb.Text.Length }
+				}
+			});
+
+			tb.Measure(new Size(300, 80));
+		}
+
+		[TestMethod]
+		public void When_Highlighter_Range_Exceeds_Text_Does_Not_Throw()
+		{
+			var tb = new TextBlock
+			{
+				Text = "abc"
+			};
+
+			tb.TextHighlighters.Add(new TextHighlighter
+			{
+				Ranges =
+				{
+					new TextRange { StartIndex = 0, Length = 999 }
+				}
+			});
+
+			tb.Measure(new Size(300, 80));
+		}
+
 		[TestMethod]
 		public void When_LineBreak_SurroundingWhiteSpace()
 		{

src/Uno.UI/UI/Xaml/Documents/UnicodeText.ICU.skia.cs

@@ -176,24 +176,32 @@ public static T GetMethod<T>()
 		public static unsafe DisposableStruct<IntPtr> CreateBiDiAndSetPara(string text, int start, int end, byte paraLevel, out IntPtr bidi, byte[]? embeddingLevels = null)
 		{
 			bidi = GetMethod<ubidi_open>()();
-			fixed (char* textPtr = &text.GetPinnableReference())
+			try
 			{
-				int setParaErrorCode;
-				if (embeddingLevels is not null)
+				fixed (char* textPtr = &text.GetPinnableReference())
 				{
-					fixed (byte* embeddingLevelsPtr = embeddingLevels)
+					int setParaErrorCode;
+					if (embeddingLevels is not null)
 					{
-						GetMethod<ubidi_setPara>()(bidi, (IntPtr)(textPtr + start), end - start, paraLevel, (IntPtr)embeddingLevelsPtr, out setParaErrorCode);
+						fixed (byte* embeddingLevelsPtr = embeddingLevels)
+						{
+							GetMethod<ubidi_setPara>()(bidi, (IntPtr)(textPtr + start), end - start, paraLevel, (IntPtr)embeddingLevelsPtr, out setParaErrorCode);
+						}
+					}
+					else
+					{
+						GetMethod<ubidi_setPara>()(bidi, (IntPtr)(textPtr + start), end - start, paraLevel, IntPtr.Zero, out setParaErrorCode);
+					}
+					if (setParaErrorCode > 0)
+					{
+						throw new InvalidOperationException($"{nameof(ubidi_setPara)} failed with error code {setParaErrorCode}");
 					}
 				}
-				else
-				{
-					GetMethod<ubidi_setPara>()(bidi, (IntPtr)(textPtr + start), end - start, paraLevel, IntPtr.Zero, out setParaErrorCode);
-				}
-				if (setParaErrorCode > 0)
-				{
-					throw new InvalidOperationException($"{nameof(ubidi_setPara)} failed with error code {setParaErrorCode}");
-				}
+			}
+			catch
+			{
+				GetMethod<ubidi_close>()(bidi);
+				throw;
 			}
 			return new DisposableStruct<IntPtr>(static bidi => GetMethod<ubidi_close>()(bidi), bidi);
 		}