From 7071c7a8d6e698e8d8684d0ed6453bf4bf08af67 Mon Sep 17 00:00:00 2001
From: Compunet <117437050+dme-compunet@users.noreply.github.com>
Date: Thu, 9 Jan 2025 15:58:16 +0200
Subject: [PATCH] Reset bidi levels of trailing whitespace after text wrapping
 (According to TR9 guidelines) (#17924)

* Reset bidi levels of trailing whitespaces to paragraph embedding level after text wrapping

* Added unit tests
---
 .../Media/TextFormatting/ShapedBuffer.cs      |  4 +-
 .../Media/TextFormatting/TextFormatterImpl.cs | 85 +++++++++++++++++++
 .../TextFormatting/TextFormatterTests.cs      | 81 ++++++++++++++++++
 3 files changed, 169 insertions(+), 1 deletion(-)

diff --git a/src/Avalonia.Base/Media/TextFormatting/ShapedBuffer.cs b/src/Avalonia.Base/Media/TextFormatting/ShapedBuffer.cs
index 3f26d081b0..65b89c1ed7 100644
--- a/src/Avalonia.Base/Media/TextFormatting/ShapedBuffer.cs
+++ b/src/Avalonia.Base/Media/TextFormatting/ShapedBuffer.cs
@@ -49,7 +49,7 @@ namespace Avalonia.Media.TextFormatting
         /// <summary>
         /// The buffer's bidi level.
         /// </summary>
-        public sbyte BidiLevel { get; }
+        public sbyte BidiLevel { get; private set; }
 
         /// <summary>
         /// The buffer's reading direction.
@@ -169,6 +169,8 @@ namespace Avalonia.Media.TextFormatting
             return new SplitResult<ShapedBuffer>(first, second);
         }
 
+        internal void ResetBidiLevel(sbyte paragraphEmbeddingLevel) => BidiLevel = paragraphEmbeddingLevel;
+
         int IReadOnlyCollection<GlyphInfo>.Count => _glyphInfos.Length;
 
         IEnumerator IEnumerable.GetEnumerator() => GetEnumerator();       
diff --git a/src/Avalonia.Base/Media/TextFormatting/TextFormatterImpl.cs b/src/Avalonia.Base/Media/TextFormatting/TextFormatterImpl.cs
index 8e2325fb14..784ee835b4 100644
--- a/src/Avalonia.Base/Media/TextFormatting/TextFormatterImpl.cs
+++ b/src/Avalonia.Base/Media/TextFormatting/TextFormatterImpl.cs
@@ -908,6 +908,11 @@ namespace Avalonia.Media.TextFormatting
                     textLineBreak = null;
                 }
 
+                if (postSplitRuns?.Count > 0)
+                {
+                    ResetTrailingWhitespaceBidiLevels(preSplitRuns, paragraphProperties.FlowDirection, objectPool);
+                }
+
                 var textLine = new TextLineImpl(preSplitRuns.ToArray(), firstTextSourceIndex, measuredLength,
                     paragraphWidth, paragraphProperties, resolvedFlowDirection,
                     textLineBreak);
@@ -923,6 +928,86 @@ namespace Avalonia.Media.TextFormatting
             }
         }
 
+        private static void ResetTrailingWhitespaceBidiLevels(RentedList<TextRun> lineTextRuns, FlowDirection paragraphFlowDirection, FormattingObjectPool objectPool)
+        {
+            if (lineTextRuns.Count == 0)
+            {
+                return;
+            }
+
+            var lastTextRunIndex = lineTextRuns.Count - 1;
+
+            var lastTextRun = lineTextRuns[lastTextRunIndex];
+
+            if (lastTextRun is not ShapedTextRun shapedText)
+            {
+                return;
+            }
+
+            var paragraphEmbeddingLevel = (sbyte)paragraphFlowDirection;
+
+            if (shapedText.BidiLevel == paragraphEmbeddingLevel)
+            {
+                return;
+            }
+
+            var textSpan = shapedText.Text.Span;
+
+            if (textSpan.IsEmpty)
+            {
+                return;
+            }
+
+            var whitespaceCharactersCount = 0;
+
+            for (var i = textSpan.Length - 1; i >= 0; i--)
+            {
+                var isWhitespace = Codepoint.ReadAt(textSpan, i, out _).IsWhiteSpace;
+
+                if (isWhitespace)
+                {
+                    whitespaceCharactersCount++;
+                }
+                else
+                {
+                    break;
+                }
+            }
+
+            if (whitespaceCharactersCount == 0)
+            {
+                return;
+            }
+
+            var splitIndex = shapedText.Length - whitespaceCharactersCount;
+
+            var (textRuns, trailingWhitespaceRuns) = SplitTextRuns([shapedText], splitIndex, objectPool);
+
+            try
+            {
+                if (trailingWhitespaceRuns != null)
+                {
+                    for (var i = 0; i < trailingWhitespaceRuns.Count; i++)
+                    {
+                        if (trailingWhitespaceRuns[i] is ShapedTextRun shapedTextRun)
+                        {
+                            shapedTextRun.ShapedBuffer.ResetBidiLevel(paragraphEmbeddingLevel);
+                        }
+                    }
+
+                    lineTextRuns.RemoveAt(lastTextRunIndex);
+
+                    lineTextRuns.AddRange(textRuns);
+                    lineTextRuns.AddRange(trailingWhitespaceRuns);
+                }
+            }
+            finally
+            {
+                objectPool.TextRunLists.Return(ref textRuns);
+                objectPool.TextRunLists.Return(ref trailingWhitespaceRuns);
+            }
+        }
+
         private struct TextRunEnumerator
         {
             private readonly ITextSource _textSource;
diff --git a/tests/Avalonia.Skia.UnitTests/Media/TextFormatting/TextFormatterTests.cs b/tests/Avalonia.Skia.UnitTests/Media/TextFormatting/TextFormatterTests.cs
index 5ff0a45e39..59c9216aec 100644
--- a/tests/Avalonia.Skia.UnitTests/Media/TextFormatting/TextFormatterTests.cs
+++ b/tests/Avalonia.Skia.UnitTests/Media/TextFormatting/TextFormatterTests.cs
@@ -168,6 +168,87 @@ namespace Avalonia.Skia.UnitTests.Media.TextFormatting
             }
         }
 
+        [Fact]
+        public void Should_Reset_Bidi_Levels_Of_Trailing_Whitespaces_After_TextWrapping()
+        {
+            using (Start())
+            {
+                const string text = "aaa bbb";
+
+                var defaultProperties = new GenericTextRunProperties(Typeface.Default);
+
+                var paragraphProperties = new GenericTextParagraphProperties(FlowDirection.RightToLeft, TextAlignment.Right, true,
+                                                                             true, defaultProperties, TextWrapping.Wrap, 0, 0, 0);
+
+                var textSource = new SimpleTextSource(text, defaultProperties);
+
+                var formatter = new TextFormatterImpl();
+
+                var firstLine = formatter.FormatLine(textSource, 0, 50, paragraphProperties);
+
+                Assert.NotNull(firstLine);
+
+                Assert.Equal(2, firstLine.TextRuns.Count);
+
+                var first = firstLine.TextRuns[0] as ShapedTextRun;
+
+                var second = firstLine.TextRuns[1] as ShapedTextRun;
+
+                Assert.NotNull(first);
+
+                Assert.NotNull(second);
+
+                Assert.Equal(" ", first.Text.ToString());
+
+                Assert.Equal("aaa", second.Text.ToString());
+
+                Assert.Equal(1, first.BidiLevel);
+
+                Assert.Equal(2, second.BidiLevel);
+            }
+        }
+
+
+        [Fact]
+        public void Should_Reset_Bidi_Levels_Of_Trailing_Whitespaces_After_TextWrapping_2()
+        {
+            using (Start())
+            {
+                const string text = "אאא בבב";
+
+                var defaultProperties = new GenericTextRunProperties(Typeface.Default);
+
+                var paragraphProperties = new GenericTextParagraphProperties(FlowDirection.LeftToRight, TextAlignment.Left, true,
+                                                                             true, defaultProperties, TextWrapping.Wrap, 0, 0, 0);
+
+                var textSource = new SimpleTextSource(text, defaultProperties);
+
+                var formatter = new TextFormatterImpl();
+
+                var firstLine = formatter.FormatLine(textSource, 0, 40, paragraphProperties);
+
+                Assert.NotNull(firstLine);
+
+                Assert.Equal(2, firstLine.TextRuns.Count);
+
+                var first = firstLine.TextRuns[0] as ShapedTextRun;
+
+                var second = firstLine.TextRuns[1] as ShapedTextRun;
+
+                Assert.NotNull(first);
+
+                Assert.NotNull(second);
+
+                Assert.Equal("אאא", first.Text.ToString());
+
+                Assert.Equal(" ", second.Text.ToString());
+
+                Assert.Equal(1, first.BidiLevel);
+
+                Assert.Equal(0, second.BidiLevel);
+            }
+        }
+
         [Fact]
         public void Should_Format_TextRuns_With_TextRunStyles()
         {