Added unit tests for TextBox to verify MaxLines property assigns the ScrollViewer.MaxHeight based on line height.

tests/Avalonia.Controls.UnitTests/TextBoxTests.cs

@@ -15,6 +15,7 @@ using Avalonia.Platform;
 using Avalonia.Rendering;
 using Avalonia.Rendering.Composition;
 using Avalonia.UnitTests;
+using Avalonia.VisualTree;
 using Moq;
 using Xunit;
 
@@ -916,6 +917,82 @@ namespace Avalonia.Controls.UnitTests
             }
         }
 
+        [Theory]
+        [InlineData(1)]
+        [InlineData(2)]
+        [InlineData(3)]
+        public void MaxLines_Sets_ScrollViewer_MaxHeight(int maxLines)
+        {
+            using (UnitTestApplication.Start(Services))
+            {
+                var target = new TextBox
+                {
+                    Template = CreateTemplate(),
+                    MaxLines = maxLines,
+
+                    // Define explicit whole number line height for predictable calculations
+                    LineHeight = 20
+                };
+
+                var impl = CreateMockTopLevelImpl();
+                var topLevel = new TestTopLevel(impl.Object)
+                {
+                    Template = CreateTopLevelTemplate(),
+                    Content = target
+                };
+                topLevel.ApplyTemplate();
+                topLevel.LayoutManager.ExecuteInitialLayoutPass();
+
+                var textPresenter = target.FindDescendantOfType<TextPresenter>();
+                Assert.Equal("PART_TextPresenter", textPresenter.Name);
+                Assert.Equal(new Thickness(0), textPresenter.Margin); // Test assumes no margin on TextPresenter
+
+                var scrollViewer = target.FindDescendantOfType<ScrollViewer>();
+                Assert.Equal("PART_ScrollViewer", scrollViewer.Name);
+                Assert.Equal(maxLines * target.LineHeight, scrollViewer.MaxHeight);
+            }
+        }
+
+        [Theory]
+        [InlineData(1)]
+        [InlineData(2)]
+        [InlineData(3)]
+        public void MaxLines_Sets_ScrollViewer_MaxHeight_With_TextPresenter_Margin(int maxLines)
+        {
+            using (UnitTestApplication.Start(Services))
+            {
+                var target = new TextBox
+                {
+                    Template = CreateTemplate(),
+                    MaxLines = maxLines,
+
+                    // Define explicit whole number line height for predictable calculations
+                    LineHeight = 20
+                };
+
+                var impl = CreateMockTopLevelImpl();
+                var topLevel = new TestTopLevel(impl.Object)
+                {
+                    Template = CreateTopLevelTemplate(),
+                    Content = target
+                };
+                topLevel.ApplyTemplate();
+                topLevel.LayoutManager.ExecuteInitialLayoutPass();
+
+                var textPresenter = target.FindDescendantOfType<TextPresenter>();
+                Assert.Equal("PART_TextPresenter", textPresenter.Name);
+                var textPresenterMargin = new Thickness(horizontal: 0, vertical: 3);
+                textPresenter.Margin = textPresenterMargin;
+
+                target.InvalidateMeasure();
+                target.Measure(Size.Infinity);
+
+                var scrollViewer = target.FindDescendantOfType<ScrollViewer>();
+                Assert.Equal("PART_ScrollViewer", scrollViewer.Name);
+                Assert.Equal((maxLines * target.LineHeight) + textPresenterMargin.Top + textPresenterMargin.Bottom, scrollViewer.MaxHeight);
+            }
+        }
+
         [Fact]
         public void CanUndo_CanRedo_Is_False_When_Initialized()
         {
@@ -1125,7 +1202,7 @@ namespace Avalonia.Controls.UnitTests
             return new FuncControlTemplate<TextBox>((control, scope) =>
             new ScrollViewer
             {
-                Name = "Part_ScrollViewer",
+                Name = "PART_ScrollViewer",
                 Template = new FuncControlTemplate<ScrollViewer>(ScrollViewerTests.CreateTemplate),
                 Content = new TextPresenter
                 {