TextLayout: TextStyleOverrides aren't applied correctly (#17922)

* Added FormattedTextSource->GetTextRun failing test

* Ensure that TextLayout-TextRuns are correctly generated from the passed TextStyleOverrides using the FormattedTextSource->GetTextRun method.

---------

Co-authored-by: Benedikt Stebner <Gillibald@users.noreply.github.com>

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

@@ -58,7 +58,6 @@ namespace Avalonia.Media.TextFormatting
 
             var currentProperties = defaultProperties;
 
-            var hasOverride = false;
 
             var i = 0;
 
@@ -92,15 +91,8 @@ namespace Avalonia.Media.TextFormatting
                 }
 
                 length = Math.Max(0, textRange.Start + textRange.Length - firstTextSourceIndex);
-
-                if (hasOverride)
-                {
-                    continue;
-                }
-
-                hasOverride = true;
-
                 currentProperties = propertiesOverride.Value;
+                break;
             }
 
             if (length < text.Length && i == textModifier.Count)

tests/Avalonia.Base.UnitTests/Media/TextFormatting/FormattedTextSourceTests.cs

@@ -0,0 +1,32 @@
+using System.Collections.Generic;
+using Avalonia.Media;
+using Avalonia.Media.TextFormatting;
+using Avalonia.Utilities;
+using Xunit;
+
+namespace Avalonia.Base.UnitTests.Media.TextFormatting
+{
+
+    public class FormattedTextSourceTests
+    {
+        [Fact]
+        public void GetTextRun_WithTwoTextStyleOverrides_ShouldGenerateCorrectFirstRun()
+        {
+            //Prepare a sample text: The two "He" at the beginning of each line should be displayed with other TextRunProperties
+            string text = "Hello World\r\nHello";
+            Typeface typeface = new Typeface();
+            GenericTextRunProperties defaultTextRunProperties = new GenericTextRunProperties(typeface);
+            IReadOnlyList<ValueSpan<TextRunProperties>> textStyleOverrides = new List<ValueSpan<TextRunProperties>>()
+            {
+                new ValueSpan<TextRunProperties>(0, 2, new GenericTextRunProperties(typeface, backgroundBrush: Brushes.Aqua)),
+                new ValueSpan<TextRunProperties>(13, 2, new GenericTextRunProperties(typeface, backgroundBrush: Brushes.Aqua)),
+            };
+
+            FormattedTextSource textSource = new FormattedTextSource(text, defaultTextRunProperties, textStyleOverrides);
+            TextRun textRun = textSource.GetTextRun(0);
+
+            Assert.Equal(2, textRun.Length);
+            Assert.Equal("He", textRun.Text.ToString());
+        }
+    }
+}