Fix TextLineIImpl.GetTextBounds for clustered trailing zero width characters (#19208) (#19251)

* Make sure we only apply the cluster offset if we are inside the current cluster

* Better naming

* Guard coveredLength against invalid values

---------

Co-authored-by: Max Katz <maxkatz6@outlook.com>

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

@@ -706,6 +706,11 @@ namespace Avalonia.Media.TextFormatting
 
                 lastBounds = currentBounds;
 
+                if(coveredLength <= 0)
+                {
+                    throw new InvalidOperationException("Covered length must be greater than zero.");
+                }
+
                 remainingLength -= coveredLength;
             }
 
@@ -1090,7 +1095,8 @@ namespace Avalonia.Media.TextFormatting
             var endHit = currentRun.GlyphRun.GetCharacterHitFromDistance(endOffset, out _);
 
             //Adjust characterLength by the cluster offset to only cover the remaining length of the cluster.
-            var characterLength = Math.Abs(startHit.FirstCharacterIndex + startHit.TrailingLength - endHit.FirstCharacterIndex - endHit.TrailingLength) - clusterOffset;
+            var characterLength = Math.Max(0, Math.Abs(startHit.FirstCharacterIndex + startHit.TrailingLength -
+                 endHit.FirstCharacterIndex - endHit.TrailingLength) - clusterOffset);
 
             if (characterLength == 0 && currentRun.Text.Length > 0 && startIndex < currentRun.Text.Length)
             {
@@ -1172,7 +1178,8 @@ namespace Avalonia.Media.TextFormatting
                 startIndex -= clusterOffset;
             }
 
-            var characterLength = Math.Abs(startHit.FirstCharacterIndex + startHit.TrailingLength - endHit.FirstCharacterIndex - endHit.TrailingLength) - clusterOffset;
+            var characterLength = Math.Max(0, Math.Abs(startHit.FirstCharacterIndex + startHit.TrailingLength - 
+                endHit.FirstCharacterIndex - endHit.TrailingLength) - clusterOffset);
 
             if (characterLength == 0 && currentRun.Text.Length > 0 && startIndex < currentRun.Text.Length)
             {

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

@@ -1158,7 +1158,7 @@ namespace Avalonia.Skia.UnitTests.Media.TextFormatting
             }
         }
         
-        private class ListTextSource : ITextSource
+        internal class ListTextSource : ITextSource
         {
             private readonly List<TextRun> _runs;
 

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

@@ -1715,6 +1715,42 @@ namespace Avalonia.Skia.UnitTests.Media.TextFormatting
             }
         }
 
+        [Fact]
+        public void Should_GetTextBounds_For_Clustered_Zero_Width_Characters()
+        {
+            const string text = "\r\n";
+
+            using (Start())
+            {
+                var defaultProperties = new GenericTextRunProperties(Typeface.Default);
+
+                var textSource = new TextFormatterTests.ListTextSource(new TextHidden(1) ,new TextCharacters(text, defaultProperties));
+
+                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);
+
+                var textBounds = textLine.GetTextBounds(2, 1);
+
+                Assert.NotEmpty(textBounds);
+
+                var firstBounds = textBounds[0];
+
+                Assert.NotEmpty(firstBounds.TextRunBounds);
+
+                var firstRunBounds = firstBounds.TextRunBounds[0];
+
+                Assert.Equal(2, firstRunBounds.TextSourceCharacterIndex);
+
+                Assert.Equal(1, firstRunBounds.Length);
+            }
+        }
+
         private class FixedRunsTextSource : ITextSource
         {
             private readonly IReadOnlyList<TextRun> _textRuns;