[Text] Fix hit testing issues (#13155)

* Repro for Line_Formatting_For_Oversized_Embedded_Runs_Does_Not_Freeze

* Repro for Line_With_IncrementalTab_Should_Return_Correct_Backspace_Position

* Fix GetBackspaceCaretCharacterHit
Fix GetPreviousCaretCharacterHit
Fix WrapWithOverflow for not text runs

* Fix custom font manager

* Move DejaVuSans to a different location to prevent using it as a fallback

---------

Co-authored-by: Nikita Tsukanov <keks9n@gmail.com>

src/Avalonia.Base/Media/GlyphRun.cs

@@ -424,14 +424,20 @@ namespace Avalonia.Media
         /// </returns>
         public CharacterHit GetPreviousCaretCharacterHit(CharacterHit characterHit)
         {
-            var previousCharacterHit = FindNearestCharacterHit(characterHit.FirstCharacterIndex - 1, out _);
+            //Always produce a hit that is on the left edge
 
-            if (characterHit.TrailingLength != 0)
+            if (characterHit.TrailingLength > 0)
             {
-                return previousCharacterHit;
+                var previousCharacterHit = FindNearestCharacterHit(characterHit.FirstCharacterIndex, out _);
+
+                return new CharacterHit(previousCharacterHit.FirstCharacterIndex);
             }
+            else
+            {
+                var previousCharacterHit = FindNearestCharacterHit(characterHit.FirstCharacterIndex - 1, out _);
 
-            return new CharacterHit(previousCharacterHit.FirstCharacterIndex);
+                return new CharacterHit(previousCharacterHit.FirstCharacterIndex);
+            }
         }
 
         /// <summary>

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

@@ -713,9 +713,31 @@ namespace Avalonia.Media.TextFormatting
 
             var measuredLength = MeasureLength(textRuns, paragraphWidth);
 
-            if(measuredLength == 0)
+            if(measuredLength == 0 && paragraphProperties.TextWrapping == TextWrapping.WrapWithOverflow)
             {
+                for (int i = 0; i < textRuns.Count; i++)
+                {
+                    measuredLength += textRuns[i].Length;
+                }
+
+                TextLineBreak? textLineBreak;
+
+                if (currentLineBreak?.TextEndOfLine is { } textEndOfLine)
+                {
+                    textLineBreak = new TextLineBreak(textEndOfLine, resolvedFlowDirection);
+                }
+                else
+                {
+                    textLineBreak = null;
+                }
 
+                var textLine = new TextLineImpl(textRuns.ToArray(), firstTextSourceIndex, measuredLength,
+                  paragraphWidth, paragraphProperties, resolvedFlowDirection,
+                  textLineBreak);
+
+                textLine.FinalizeLine();
+
+                return textLine;
             }
 
             var currentLength = 0;

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

@@ -1,5 +1,6 @@
 using System;
 using System.Collections.Generic;
+using Avalonia.Media.TextFormatting.Unicode;
 using Avalonia.Utilities;
 
 namespace Avalonia.Media.TextFormatting
@@ -567,82 +568,13 @@ namespace Avalonia.Media.TextFormatting
         /// <inheritdoc/>
         public override CharacterHit GetPreviousCaretCharacterHit(CharacterHit characterHit)
         {
-            if (_textRuns.Length == 0 || _indexedTextRuns is null)
-            {
-                return new CharacterHit();
-            }
-
-            if (characterHit.TrailingLength > 0 && characterHit.FirstCharacterIndex <= FirstTextSourceIndex)
-            {
-                return new CharacterHit(FirstTextSourceIndex);
-            }
-
-            var characterIndex = characterHit.FirstCharacterIndex + characterHit.TrailingLength;
-
-            if (characterIndex <= FirstTextSourceIndex)
-            {
-                return new CharacterHit(FirstTextSourceIndex);
-            }
-
-            var currentCharacterrHit = characterHit;
-
-            var currentRun = GetRunAtCharacterIndex(characterIndex, LogicalDirection.Backward, out var currentPosition);
-
-            if (currentPosition == characterHit.FirstCharacterIndex)
-            {
-                currentRun = GetRunAtCharacterIndex(characterHit.FirstCharacterIndex, LogicalDirection.Backward, out currentPosition);
-            }
-
-            var previousCharacterHit = characterHit;
-
-            switch (currentRun)
-            {
-                case ShapedTextRun shapedRun:
-                    {
-                        var offset = Math.Max(0, currentPosition - shapedRun.GlyphRun.Metrics.FirstCluster);
-
-                        if (offset > 0)
-                        {
-                            currentCharacterrHit = new CharacterHit(Math.Max(0, characterHit.FirstCharacterIndex - offset), characterHit.TrailingLength);
-                        }
-
-                        previousCharacterHit = shapedRun.GlyphRun.GetPreviousCaretCharacterHit(currentCharacterrHit);
-
-                        if (offset > 0)
-                        {
-                            previousCharacterHit = new CharacterHit(previousCharacterHit.FirstCharacterIndex + offset, previousCharacterHit.TrailingLength);
-                        }
-                        break;
-                    }
-                case TextRun:
-                    {
-                        if (characterHit.TrailingLength > 0)
-                        {
-                            previousCharacterHit = new CharacterHit(currentPosition, currentRun.Length);
-
-                        }
-                        else
-                        {
-                            previousCharacterHit = new CharacterHit(currentPosition + currentRun.Length);
-                        }
-
-                        break;
-                    }
-            }
-
-            if (characterIndex == previousCharacterHit.FirstCharacterIndex + previousCharacterHit.TrailingLength)
-            {
-                return characterHit;
-            }
-
-            return previousCharacterHit;
+            return GetPreviousCharacterHit(characterHit, false);
         }
 
         /// <inheritdoc/>
         public override CharacterHit GetBackspaceCaretCharacterHit(CharacterHit characterHit)
         {
-            // same operation as move-to-previous
-            return GetPreviousCaretCharacterHit(characterHit);
+            return GetPreviousCharacterHit(characterHit, true);
         }
 
         public override IReadOnlyList<TextBounds> GetTextBounds(int firstTextSourceIndex, int textLength)
@@ -823,6 +755,95 @@ namespace Avalonia.Media.TextFormatting
             return result;
         }
 
+        private CharacterHit GetPreviousCharacterHit(CharacterHit characterHit, bool useGraphemeBoundaries)
+        {
+            if (_textRuns.Length == 0 || _indexedTextRuns is null)
+            {
+                return new CharacterHit();
+            }
+
+            if (characterHit.TrailingLength > 0 && characterHit.FirstCharacterIndex <= FirstTextSourceIndex)
+            {
+                return new CharacterHit(FirstTextSourceIndex);
+            }
+
+            var characterIndex = characterHit.FirstCharacterIndex + characterHit.TrailingLength;
+
+            if (characterIndex <= FirstTextSourceIndex)
+            {
+                return new CharacterHit(FirstTextSourceIndex);
+            }
+
+            var currentCharacterHit = characterHit;
+
+            var currentRun = GetRunAtCharacterIndex(characterIndex, LogicalDirection.Backward, out var currentPosition);
+
+            var previousCharacterHit = characterHit;
+
+            switch (currentRun)
+            {
+                case ShapedTextRun shapedRun:
+                    {
+                        var offset = Math.Max(0, currentPosition - shapedRun.GlyphRun.Metrics.FirstCluster);
+
+                        if (offset > 0)
+                        {
+                            currentCharacterHit = new CharacterHit(Math.Max(0, characterHit.FirstCharacterIndex - offset), characterHit.TrailingLength);
+                        }
+
+                        previousCharacterHit = shapedRun.GlyphRun.GetPreviousCaretCharacterHit(currentCharacterHit);
+
+                        if (useGraphemeBoundaries)
+                        {
+                            var textPosition = Math.Max(0, previousCharacterHit.FirstCharacterIndex - shapedRun.GlyphRun.Metrics.FirstCluster);
+
+                            var text = shapedRun.GlyphRun.Characters.Slice(textPosition);
+
+                            var graphemeEnumerator = new GraphemeEnumerator(text.Span);
+
+                            var length = 0;
+
+                            var clusterLength = Math.Max(0, currentCharacterHit.FirstCharacterIndex + currentCharacterHit.TrailingLength - 
+                                previousCharacterHit.FirstCharacterIndex - previousCharacterHit.TrailingLength);
+
+                            while (graphemeEnumerator.MoveNext(out var grapheme))
+                            {
+                                if (length + grapheme.Length < clusterLength)
+                                {
+                                    length += grapheme.Length;
+
+                                    continue;
+                                }
+
+                                previousCharacterHit = new CharacterHit(previousCharacterHit.FirstCharacterIndex + length);
+
+                                break;
+                            }
+                        }
+
+                        if (offset > 0)
+                        {
+                            previousCharacterHit = new CharacterHit(previousCharacterHit.FirstCharacterIndex + offset, previousCharacterHit.TrailingLength);
+                        }
+
+                        break;
+                    }
+                case TextRun:
+                    {
+                        previousCharacterHit = new CharacterHit(currentPosition);
+
+                        break;
+                    }
+            }
+
+            if (characterIndex == previousCharacterHit.FirstCharacterIndex + previousCharacterHit.TrailingLength)
+            {
+                return characterHit;
+            }
+
+            return previousCharacterHit;
+        }
+
         private TextBounds GetTextRunBoundsRightToLeft(int firstRunIndex, int lastRunIndex, double endX,
             int firstTextSourceIndex, int currentPosition, int remainingLength, out int coveredLength, out int newPosition)
         {
@@ -1146,13 +1167,6 @@ namespace Avalonia.Media.TextFormatting
                             }
                             else
                             {
-                                if (previousRun is not null && previousRun is not ShapedTextRun && codepointIndex == textPosition + firstCluster)
-                                {
-                                    textPosition -= previousRun.Length;
-
-                                    return previousRun;
-                                }
-
                                 if (codepointIndex > firstCluster && codepointIndex <= firstCluster + currentRun.Length)
                                 {
                                     return currentRun;
@@ -1170,9 +1184,19 @@ namespace Avalonia.Media.TextFormatting
                         }
                     case TextRun:
                         {
-                            if (codepointIndex == textPosition)
+                            if(direction == LogicalDirection.Forward)
                             {
-                                return currentRun;
+                                if (textPosition == codepointIndex)
+                                {
+                                    return currentRun;
+                                }
+                            }
+                            else
+                            {
+                                if (textPosition + currentRun.Length == codepointIndex)
+                                {
+                                    return currentRun;
+                                }
                             }
 
                             if (runIndex + 1 >= _textRuns.Length)

tests/Avalonia.Skia.UnitTests/Avalonia.Skia.UnitTests.csproj

@@ -10,6 +10,8 @@
   <Import Project="..\..\build\SharedVersion.props" />
   <ItemGroup>
     <EmbeddedResource Include="..\Avalonia.RenderTests\*\*.ttf" />
+    <None Remove="Fonts\DejaVuSans.ttf" />
+    <EmbeddedResource Include="Fonts\DejaVuSans.ttf" />
   </ItemGroup>
   <ItemGroup>
     <ProjectReference Include="..\..\src\Avalonia.Base\Avalonia.Base.csproj" />

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

@@ -2,6 +2,7 @@
 using System.Collections.Generic;
 using System.Linq;
 using Avalonia.Media;
+using Avalonia.Media.Imaging;
 using Avalonia.Media.TextFormatting;
 using Avalonia.Media.TextFormatting.Unicode;
 using Avalonia.UnitTests;
@@ -765,6 +766,62 @@ namespace Avalonia.Skia.UnitTests.Media.TextFormatting
             }
         }
 
+        [Fact]
+        public void Line_Formatting_For_Oversized_Embedded_Runs_Does_Not_Produce_Empty_Lines()
+        {
+            var defaultRunProperties = new GenericTextRunProperties(Typeface.Default, foregroundBrush: Brushes.Black);
+            var paragraphProperties = new GenericTextParagraphProperties(defaultRunProperties,
+                textWrap: TextWrapping.WrapWithOverflow);
+
+            using (Start())
+            {
+                var source = new ListTextSource(new RectangleRun(new Rect(0, 0, 200, 10), Brushes.Aqua));
+                var textLine = TextFormatter.Current.FormatLine(source, 0, 100, paragraphProperties);
+                Assert.Equal(200d, textLine.WidthIncludingTrailingWhitespace);
+            }
+        }
+
+        class IncrementalTabProperties : TextParagraphProperties
+        {
+            public IncrementalTabProperties(TextRunProperties defaultTextRunProperties)
+            {
+                DefaultTextRunProperties = defaultTextRunProperties;
+            }
+
+            public override FlowDirection FlowDirection { get; }
+            public override TextAlignment TextAlignment { get; }
+            public override double LineHeight { get; }
+            public override bool FirstLineInParagraph { get; }
+            public override TextRunProperties DefaultTextRunProperties { get; }
+            public override TextWrapping TextWrapping { get; }
+            public override double Indent { get; }
+            public override double DefaultIncrementalTab => 64;
+        }
+        
+        [Fact]
+        public void Line_With_IncrementalTab_Should_Return_Correct_Backspace_Position()
+        {
+            using (Start())
+            {
+                var typeface = new Typeface(FontFamily.Parse("resm:Avalonia.Skia.UnitTests.Fonts?assembly=Avalonia.Skia.UnitTests#DejaVu Sans"));
+                
+                var defaultRunProperties = new GenericTextRunProperties(typeface, foregroundBrush: Brushes.Black);
+                var paragraphProperties = new IncrementalTabProperties(defaultRunProperties);
+
+                var text = new TextCharacters("ff",
+                    new GenericTextRunProperties(typeface, foregroundBrush: Brushes.Black));
+                
+                var source = new ListTextSource(text);
+                
+                var textLine = TextFormatter.Current.FormatLine(source, 0, double.PositiveInfinity, paragraphProperties);
+                
+                var backspaceHit = textLine.GetBackspaceCaretCharacterHit(new CharacterHit(2));
+                Assert.Equal(1, backspaceHit.FirstCharacterIndex);
+                Assert.Equal(0, backspaceHit.TrailingLength);
+            }
+            
+        }
+
         protected readonly record struct SimpleTextSource : ITextSource
         {
             private readonly string _text;

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

@@ -469,18 +469,18 @@ namespace Avalonia.Skia.UnitTests.Media.TextFormatting
 
                 var currentHit = textLine.GetPreviousCaretCharacterHit(new CharacterHit(3, 1));
 
-                Assert.Equal(2, currentHit.FirstCharacterIndex);
-                Assert.Equal(1, currentHit.TrailingLength);
+                Assert.Equal(3, currentHit.FirstCharacterIndex);
+                Assert.Equal(0, currentHit.TrailingLength);
 
                 currentHit = textLine.GetPreviousCaretCharacterHit(currentHit);
 
-                Assert.Equal(1, currentHit.FirstCharacterIndex);
-                Assert.Equal(1, currentHit.TrailingLength);
+                Assert.Equal(2, currentHit.FirstCharacterIndex);
+                Assert.Equal(0, currentHit.TrailingLength);
 
                 currentHit = textLine.GetPreviousCaretCharacterHit(currentHit);
 
-                Assert.Equal(0, currentHit.FirstCharacterIndex);
-                Assert.Equal(1, currentHit.TrailingLength);
+                Assert.Equal(1, currentHit.FirstCharacterIndex);
+                Assert.Equal(0, currentHit.TrailingLength);
 
                 currentHit = textLine.GetPreviousCaretCharacterHit(currentHit);
 
@@ -786,21 +786,21 @@ namespace Avalonia.Skia.UnitTests.Media.TextFormatting
 
                 var characterHit = textLine.GetPreviousCaretCharacterHit(new CharacterHit(20, 1));
 
-                Assert.Equal(19, characterHit.FirstCharacterIndex);
+                Assert.Equal(20, characterHit.FirstCharacterIndex);
 
-                Assert.Equal(1, characterHit.TrailingLength);
+                Assert.Equal(0, characterHit.TrailingLength);
 
                 characterHit = textLine.GetPreviousCaretCharacterHit(new CharacterHit(10, 1));
 
-                Assert.Equal(9, characterHit.FirstCharacterIndex);
+                Assert.Equal(10, characterHit.FirstCharacterIndex);
 
-                Assert.Equal(1, characterHit.TrailingLength);
+                Assert.Equal(0, characterHit.TrailingLength);
 
                 characterHit = textLine.GetPreviousCaretCharacterHit(characterHit);
 
-                Assert.Equal(8, characterHit.FirstCharacterIndex);
+                Assert.Equal(9, characterHit.FirstCharacterIndex);
 
-                Assert.Equal(1, characterHit.TrailingLength);
+                Assert.Equal(0, characterHit.TrailingLength);
 
                 characterHit = textLine.GetPreviousCaretCharacterHit(new CharacterHit(21));