fix(text): preserve lines at rounded fractional heights (#21834)

* test(textblock): cover fractional layout rounding

A wrapped TextBlock with a fractional line height must retain every line when its desired height lands exactly on a fractional-scale pixel boundary.

* fix(text): preserve lines at rounded fractional heights

TextLayout treated a floating-point accumulation error as a MaxHeight overflow. Use the established tolerant comparison so a line that exactly fits its rounded layout bounds is retained.

---------

Co-authored-by: Julien Lebosquain <julien@lebosquain.net>

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

@@ -603,7 +603,7 @@ namespace Avalonia.Media.TextFormatting
 
                     //Fulfill max height constraint
                     if (textLines.Count > 0 && !double.IsPositiveInfinity(MaxHeight)
-                        && Height + textLine.Height > MaxHeight)
+                        && MathUtilities.GreaterThan(Height + textLine.Height, MaxHeight))
                     {
                         if (previousLine?.TextLineBreak != null && _textTrimming != TextTrimming.None)
                         {

tests/Avalonia.Controls.UnitTests/TextBlockTests.cs

@@ -536,6 +536,32 @@ namespace Avalonia.Controls.UnitTests
             Assert.Equal(new Rect(0, 0, 27.954545454545453, 14.522727272727273), target.Bounds);
         }
 
+        [Fact]
+        public void TextBlock_With_Fractional_LineHeight_Should_Not_Cull_Last_Line_At_Fractional_Scaling()
+        {
+            using var app = UnitTestApplication.Start(TestServices.MockPlatformRenderInterface);
+
+            var target = new TextBlock
+            {
+                Text = "first second third",
+                FontSize = 16,
+                LineHeight = 20.8,
+                TextWrapping = TextWrapping.Wrap,
+                Width = 50,
+                HorizontalAlignment = HorizontalAlignment.Left,
+                VerticalAlignment = VerticalAlignment.Top,
+            };
+            var root = new TestRoot(target)
+            {
+                LayoutScaling = 1.25,
+            };
+
+            root.Measure(Size.Infinity);
+            root.Arrange(new Rect(root.DesiredSize));
+
+            Assert.Equal(3, target.TextLayout.TextLines.Count);
+        }
+
         [Fact]
         public void TextBlock_With_UseLayoutRounding_False_Should_Not_Round_Padding_In_MeasureOverride()
         {