Do not shape and render null terminator (#17119)

* Ignore null terminator and replace them with a zero space before shaping

* Replace multiple null terminators at random positions

src/Avalonia.Base/Media/TextFormatting/TextCharacters.cs

@@ -9,6 +9,8 @@ namespace Avalonia.Media.TextFormatting
     /// </summary>
     public class TextCharacters : TextRun
     {
+        private static char ZeroWidthSpace = '\u200b';
+
         /// <summary>
         /// Constructs a run for text content from a string.
         /// </summary>
@@ -82,7 +84,21 @@ namespace Avalonia.Media.TextFormatting
             var previousGlyphTypeface = previousProperties?.CachedGlyphTypeface;
             var textSpan = text.Span;
 
-            if (TryGetShapeableLength(textSpan, defaultGlyphTypeface, null, out var count))
+            var count = 0;
+            var codepoints = new CodepointEnumerator(textSpan);
+
+            while(codepoints.MoveNext(out var firstCodepoint) && firstCodepoint.Value == 0)
+            {
+                count++;
+            }
+
+            //Detect null terminator
+            if (count > 0)
+            {
+                return new UnshapedTextRun(new string(ZeroWidthSpace, count).AsMemory(), defaultProperties, biDiLevel);
+            }
+
+            if (TryGetShapeableLength(textSpan, defaultGlyphTypeface, null, out count))
             {
                 return new UnshapedTextRun(text.Slice(0, count), defaultProperties.WithTypeface(defaultTypeface),
                     biDiLevel);
@@ -177,6 +193,12 @@ namespace Avalonia.Media.TextFormatting
                 var currentCodepoint = currentGrapheme.FirstCodepoint;
                 var currentScript = currentCodepoint.Script;
 
+                if(currentCodepoint.Value == 0)
+                {
+                    //Do not include null terminators
+                    break;
+                }
+
                 if (!currentCodepoint.IsWhiteSpace
                     && defaultGlyphTypeface != null
                     && defaultGlyphTypeface.TryGetGlyph(currentCodepoint, out _))

src/Skia/Avalonia.Skia/TextShaperImpl.cs

@@ -14,8 +14,6 @@ namespace Avalonia.Skia
 {
     internal class TextShaperImpl : ITextShaperImpl
     {
-        private const uint ZeroWidthSpace = '\u200b';
-
         private static readonly ConcurrentDictionary<int, Language> s_cachedLanguage = new();
 
         public ShapedBuffer ShapeText(ReadOnlyMemory<char> text, TextShaperOptions options)
@@ -69,17 +67,7 @@ namespace Avalonia.Skia
 
                     var glyphIndex = (ushort)sourceInfo.Codepoint;
 
-                    var glyphCluster = (int)(sourceInfo.Cluster);
-
-                    if (glyphIndex == 0)
-                    {
-                        var codepoint = Codepoint.ReadAt(textSpan, glyphCluster, out _);
-
-                        if (codepoint.GeneralCategory == GeneralCategory.Control)
-                        {
-                            glyphIndex = options.Typeface.GetGlyph(ZeroWidthSpace);
-                        }
-                    }
+                    var glyphCluster = (int)sourceInfo.Cluster;
 
                     var glyphAdvance = GetGlyphAdvance(glyphPositions, i, textScale) + options.LetterSpacing;
 

tests/Avalonia.Skia.UnitTests/Media/TextFormatting/TextLineTests.cs

@@ -1374,6 +1374,31 @@ namespace Avalonia.Skia.UnitTests.Media.TextFormatting
             }
         }
 
+        [Theory]
+        [InlineData("\0", 0.0)]
+        [InlineData("\0\0\0", 0.0)]
+        [InlineData("\0A\0\0", 7.201171875)]
+        [InlineData("\0AA\0AA\0", 28.8046875)]
+        public void Should_Ignore_Null_Terminator(string text, double width)
+        {
+            using (Start())
+            {
+                var defaultProperties = new GenericTextRunProperties(Typeface.Default);
+                var textSource = new SingleBufferTextSource(text, defaultProperties, true);
+
+                var formatter = new TextFormatterImpl();
+
+                var textLine =
+                    formatter.FormatLine(textSource, 0, double.PositiveInfinity,
+                        new GenericTextParagraphProperties(FlowDirection.LeftToRight, TextAlignment.Left,
+                        true, true, defaultProperties, TextWrapping.NoWrap, 0, 0, 0));
+
+                Assert.NotNull(textLine);
+
+                Assert.Equal(width, textLine.Width);
+            }
+        }
+
         private class FixedRunsTextSource : ITextSource
         {
             private readonly IReadOnlyList<TextRun> _textRuns;