Merge pull request #12589 from Actipro/fix-textbox-padding

Fixes TextBox measure logic for MaxLines scenario

src/Avalonia.Controls/TextBox.cs

@@ -1879,6 +1879,37 @@ namespace Avalonia.Controls
             return text.Substring(start, end - start);
         }
 
+        /// <summary>
+        /// Returns the sum of any vertical whitespace added between the <see cref="ScrollViewer"/> and <see cref="TextPresenter"/> in the control template.
+        /// </summary>
+        /// <returns>The total vertical whitespace.</returns>
+        private double GetVerticalSpaceBetweenScrollViewerAndPresenter()
+        {
+            var verticalSpace = 0.0;
+            if (_presenter != null)
+            {
+                Visual? visual = _presenter;
+                while ((visual != null) && (visual != this))
+                {
+                    if (visual == _scrollViewer)
+                    {
+                        // ScrollViewer is a stopping point and should only include the Padding
+                        verticalSpace += _scrollViewer.Padding.Top + _scrollViewer.Padding.Bottom;
+                        break;
+                    }
+
+                    var margin = visual.GetValue<Thickness>(Layoutable.MarginProperty);
+                    var padding = visual.GetValue<Thickness>(Decorator.PaddingProperty);
+                    
+                    verticalSpace += margin.Top + padding.Top + padding.Bottom + margin.Bottom;
+
+                    visual = visual.VisualParent;
+                }
+            }
+
+            return verticalSpace;
+        }
+
         /// <summary>
         /// Raises both the <see cref="TextChanging"/> and <see cref="TextChanged"/> events.
         /// </summary>
@@ -2032,8 +2063,9 @@ namespace Avalonia.Controls
                     var typeface = new Typeface(FontFamily, FontStyle, FontWeight, FontStretch);
                     var paragraphProperties = TextLayout.CreateTextParagraphProperties(typeface, fontSize, null, default, default, null, default, LineHeight, default);
                     var textLayout = new TextLayout(new MaxLinesTextSource(MaxLines), paragraphProperties);
+                    var verticalSpace = GetVerticalSpaceBetweenScrollViewerAndPresenter();
 
-                    maxHeight = Math.Ceiling(textLayout.Height);
+                    maxHeight = Math.Ceiling(textLayout.Height + verticalSpace);
                 }
 
                 _scrollViewer.SetCurrentValue(MaxHeightProperty, maxHeight);

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
                 {