Preserve run formatting when applying SelectionForegroundBrush in SelectableTextBlock (#20110)

* Add failing test for preserving run formatting during selection

* Fix selection styling to preserve original text formatting in SelectableTextBlock

* fix: apply old logic when `_textRuns` is null

src/Avalonia.Controls/SelectableTextBlock.cs

@@ -197,7 +197,7 @@ namespace Avalonia.Controls
                 LineSpacing = LineSpacing
             };
 
-            IReadOnlyList<ValueSpan<TextRunProperties>>? textStyleOverrides = null;
+            List<ValueSpan<TextRunProperties>>? textStyleOverrides = null;
             var selectionStart = SelectionStart;
             var selectionEnd = SelectionEnd;
             var start = Math.Min(selectionStart, selectionEnd);
@@ -205,12 +205,52 @@ namespace Avalonia.Controls
 
             if (length > 0 && SelectionForegroundBrush != null)
             {
-                textStyleOverrides = new[]
+                if (_textRuns != null)
                 {
+                    // Apply selection foreground color without changing the original text formatting.
+                    // The built-in SelectableTextBlock selection logic recreates TextRunProperties,
+                    // which overwrites run-specific Typeface/FontFeatures/FontSize and breaks bold/italic.
+                    // Here we reuse each run's existing properties and only override the foreground brush.
+
+                    var accumulatedLength = 0;
+                    foreach (var textRun in _textRuns)
+                    {
+                        var runLength = textRun.Text.Length;
+                        if (accumulatedLength + runLength <= start ||
+                            accumulatedLength >= start + length)
+                        {
+                            accumulatedLength += runLength;
+                            continue;
+                        }
+
+                        var overlapStart = Math.Max(start, accumulatedLength);
+                        var overlapEnd = Math.Min(start + length, accumulatedLength + runLength);
+                        var overlapLength = overlapEnd - overlapStart;
+
+                        textStyleOverrides ??= [];
+
+                        textStyleOverrides.Add(
+                            new ValueSpan<TextRunProperties>(
+                                overlapStart,
+                                overlapLength,
+                                new GenericTextRunProperties(
+                                    textRun.Properties?.Typeface ?? typeface,
+                                    textRun.Properties?.FontFeatures ?? FontFeatures,
+                                    FontSize,
+                                    foregroundBrush: SelectionForegroundBrush)));
+
+                        accumulatedLength += runLength;
+                    }
+                }
+                else
+                {
+                    textStyleOverrides =
+                    [
                         new ValueSpan<TextRunProperties>(start, length,
-                        new GenericTextRunProperties(typeface, FontFeatures, FontSize,
-                            foregroundBrush: SelectionForegroundBrush))
-                    };
+                            new GenericTextRunProperties(typeface, FontFeatures, FontSize,
+                                foregroundBrush: SelectionForegroundBrush))
+                    ];
+                }
             }
 
             ITextSource textSource;

tests/Avalonia.Controls.UnitTests/SelectableTextBlockTests.cs

@@ -0,0 +1,59 @@
+using System.Linq;
+using Avalonia.Controls.Documents;
+using Avalonia.Media;
+using Avalonia.Media.TextFormatting;
+using Avalonia.UnitTests;
+using Xunit;
+
+namespace Avalonia.Controls.UnitTests
+{
+    public class SelectableTextBlockTests : ScopedTestBase
+    {
+        [Fact]
+        public void SelectionForeground_Should_Not_Reset_Run_Typeface_And_Style()
+        {
+            using (UnitTestApplication.Start(TestServices.MockPlatformRenderInterface))
+            {
+                var target = new SelectableTextBlock
+                {
+                    SelectionForegroundBrush = Brushes.Red
+                };
+
+                var run = new Run("Hello")
+                {
+                    FontWeight = FontWeight.Bold,
+                    FontStyle = FontStyle.Italic,
+                    FontSize = 20
+                };
+
+                target.Inlines.Add(run);
+
+                target.Measure(Size.Infinity);
+
+                target.SelectionStart = 0;
+                target.SelectionEnd = run.Text.Length;
+
+                target.Measure(Size.Infinity);
+
+                var textLayout = target.TextLayout;
+                Assert.NotNull(textLayout);
+
+                var textRuns = textLayout.TextLines
+                    .SelectMany(l => l.TextRuns)
+                    .OfType<ShapedTextRun>()
+                    .ToList();
+
+                Assert.NotEmpty(textRuns);
+
+                var selectedRun = textRuns[0];
+                var props = selectedRun.Properties;
+
+                Assert.Equal(FontWeight.Bold, props.Typeface.Weight);
+                Assert.Equal(FontStyle.Italic, props.Typeface.Style);
+
+                Assert.Same(target.SelectionForegroundBrush, props.ForegroundBrush);
+            }
+        }
+
+    }
+}