fix(text): avoid per-inline surrogate mutation

Author: agneszitte

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

@@ -138,7 +138,7 @@ internal unsafe UnicodeText(
 
 		foreach (var inline in inlines)
 		{
-			var inlineText = SanitizeText(inline.GetText());
+			var inlineText = inline.GetText();
 			if (string.IsNullOrEmpty(inlineText))
 			{
 				continue;
@@ -151,12 +151,36 @@ internal unsafe UnicodeText(
 
 			var currentFontDetails = inline.FontInfo;
 			int currentScript = 0;
-			for (var i = 0; i < inlineText.Length; i += char.IsSurrogate(inlineText, i) ? 2 : 1)
+			for (var i = 0; i < inlineText.Length;)
 			{
 				FontDetails newFontDetails;
-				var codepoint = char.ConvertToUtf32(inlineText, i);
+				int codepoint;
+				int codepointLength;
+				if (char.IsHighSurrogate(inlineText[i]))
+				{
+					if (i + 1 < inlineText.Length && char.IsLowSurrogate(inlineText[i + 1]))
+					{
+						codepoint = char.ConvertToUtf32(inlineText, i);
+						codepointLength = 2;
+					}
+					else
+					{
+						codepoint = '\uFFFD';
+						codepointLength = 1;
+					}
+				}
+				else if (char.IsLowSurrogate(inlineText[i]))
+				{
+					codepoint = '\uFFFD';
+					codepointLength = 1;
+				}
+				else
+				{
+					codepoint = inlineText[i];
+					codepointLength = 1;
+				}
 
-				var newScript = ICU.GetMethod<ICU.uscript_getScript>()(char.ConvertToUtf32(inlineText, i), out var errorCode);
+				var newScript = ICU.GetMethod<ICU.uscript_getScript>()(codepoint, out var errorCode);
 				ICU.CheckErrorCode<ICU.uscript_getScript>(errorCode);
 
 				if (newScript != currentScript)
@@ -185,6 +209,8 @@ internal unsafe UnicodeText(
 					}
 					currentFontDetails = newFontDetails;
 				}
+
+				i += codepointLength;
 			}
 
 			scriptBreaks.Add(inlineStart + inlineText.Length);