Fixed the issue where a newline character required two backspaces to be deleted in Windows. (#20073)

* Fixed the issue where a newline character required two backspaces to be deleted in Windows.

* add unit test

* Handling the differences between HeadlessTextShaperStub and TextShaperImpl

* Make unit tests available

* Treat "\r\n" as a single unit.

* The backspace test against CRLF

* Backspace should treat CRLF as a unit.

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

@@ -853,8 +853,13 @@ namespace Avalonia.Media.TextFormatting
                         {
                             var length = 0;
 
-                            while (Codepoint.ReadAt(shapedRun.GlyphRun.Characters.Span, length, out var count) != Codepoint.ReplacementCodepoint)
+                            while (Codepoint.ReadAt(shapedRun.GlyphRun.Characters.Span, length, out var count) is Codepoint codepoint && codepoint != Codepoint.ReplacementCodepoint)
                             {
+                                if (codepoint.Value == 0x0D && Codepoint.ReadAt(shapedRun.GlyphRun.Characters.Span, length + count, out var lfCount).Value == 0x0A)
+                                {
+                                    count += lfCount;
+                                }
+
                                 if (length + count >= runOffset)
                                 {
                                     break;
@@ -869,7 +874,7 @@ namespace Avalonia.Media.TextFormatting
                         {
                             previousCharacterHit = shapedRun.GlyphRun.GetPreviousCaretCharacterHit(new CharacterHit(firstCluster + runOffset));
 
-                            if(textSourceOffset > 0)
+                            if (textSourceOffset > 0)
                             {
                                 previousCharacterHit = new CharacterHit(textSourceOffset + previousCharacterHit.FirstCharacterIndex, previousCharacterHit.TrailingLength);
                             }

src/Headless/Avalonia.Headless/HeadlessPlatformStubs.cs

@@ -174,6 +174,12 @@ namespace Avalonia.Headless
 
                 var codepoint = Codepoint.ReadAt(textSpan, i, out var count);
 
+                // Handle CRLF as a single cluster
+                if (codepoint.Value == 0x0D && Codepoint.ReadAt(textSpan, i + count, out var lfCount).Value == 0x0A)
+                {
+                    count += lfCount;
+                }
+
                 var glyphIndex = typeface.GetGlyph(codepoint);
 
                 for (var j = 0; j < count; ++j)
@@ -218,7 +224,7 @@ namespace Avalonia.Headless
             return false;
         }
 
-        public virtual bool TryCreateGlyphTypeface(string familyName, FontStyle style, FontWeight weight, 
+        public virtual bool TryCreateGlyphTypeface(string familyName, FontStyle style, FontWeight weight,
             FontStretch stretch, [NotNullWhen(true)] out IGlyphTypeface? glyphTypeface)
         {
             glyphTypeface = null;
@@ -238,9 +244,9 @@ namespace Avalonia.Headless
         public virtual bool TryCreateGlyphTypeface(Stream stream, FontSimulations fontSimulations, out IGlyphTypeface glyphTypeface)
         {
             glyphTypeface = new HeadlessGlyphTypefaceImpl(
-                FontFamily.DefaultFontFamilyName, 
+                FontFamily.DefaultFontFamilyName,
-                fontSimulations.HasFlag(FontSimulations.Oblique) ? FontStyle.Italic : FontStyle.Normal, 
+                fontSimulations.HasFlag(FontSimulations.Oblique) ? FontStyle.Italic : FontStyle.Normal,
-                fontSimulations.HasFlag(FontSimulations.Bold) ? FontWeight.Bold : FontWeight.Normal, 
+                fontSimulations.HasFlag(FontSimulations.Bold) ? FontWeight.Bold : FontWeight.Normal,
                 FontStretch.Normal);
 
             TryCreateGlyphTypefaceCount++;

tests/Avalonia.Controls.UnitTests/TextBoxTests.cs

@@ -2129,6 +2129,25 @@ namespace Avalonia.Controls.UnitTests
             }
         }
 
+        [Fact]
+        public void Backspace_Should_Delete_CRLFNewline_Character_At_Once()
+        {
+            using var _ = UnitTestApplication.Start(Services);
+            var target = new TextBox
+            {
+                Template = CreateTemplate(),
+                Text = $"First\r\nSecond",
+                CaretIndex = 7
+            };
+            target.ApplyTemplate();
+
+            // (First\r\nSecond)
+            RaiseKeyEvent(target, Key.Back, KeyModifiers.None);
+            // (FirstSecond)
+
+            Assert.Equal("FirstSecond", target.Text);
+        }
+
         private static TestServices FocusServices => TestServices.MockThreadingInterface.With(
             keyboardDevice: () => new KeyboardDevice(),
             keyboardNavigation: () => new KeyboardNavigationHandler(),

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

@@ -2285,6 +2285,29 @@ namespace Avalonia.Skia.UnitTests.Media.TextFormatting
             }
         }
 
+        [Fact]
+        public void Backspace_Should_Treat_CRLF_As_A_Unit()
+        {
+            using (Start())
+            {
+                var typeface = new Typeface(FontFamily.Parse("resm:Avalonia.Skia.UnitTests.Fonts?assembly=Avalonia.Skia.UnitTests#Manrope"));
+                var defaultProperties = new GenericTextRunProperties(typeface);
+                var textSource = new SingleBufferTextSource("one\r\n", defaultProperties);
+
+                var formatter = new TextFormatterImpl();
+
+                var textLine =
+                    formatter.FormatLine(textSource, 0, double.PositiveInfinity,
+                        new GenericTextParagraphProperties(defaultProperties));
+
+                Assert.NotNull(textLine);
+
+                var backspaceHit = textLine.GetBackspaceCaretCharacterHit(new CharacterHit(5));
+
+                Assert.Equal(3, backspaceHit.FirstCharacterIndex);
+            }
+        }
+
         private class FixedRunsTextSource : ITextSource
         {
             private readonly IReadOnlyList<TextRun> _textRuns;