[Feature] Move LetterSpacing to TextElement as inherited attached property (#20141)

* Add LetterSpacing property to the Button

* Created attached property

* Added samples

* Updated implementation

* Updated Themes

* Updated ButtonPage LetterSpacing sample

* Updated templates

* Updated API

api/Avalonia.nupkg.xml

@@ -1,4 +1,4 @@
-<?xml version="1.0" encoding="utf-8"?>
+<?xml version="1.0" encoding="utf-8"?>
 <!-- https://learn.microsoft.com/dotnet/fundamentals/package-validation/diagnostic-ids -->
 <Suppressions xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema">
   <Suppression>
@@ -49,6 +49,18 @@
     <Left>baseline/Avalonia/lib/net10.0/Avalonia.Base.dll</Left>
     <Right>current/Avalonia/lib/net10.0/Avalonia.Base.dll</Right>
   </Suppression>
+  <Suppression>
+    <DiagnosticId>CP0002</DiagnosticId>
+    <Target>F:Avalonia.Controls.TextBlock.LetterSpacingProperty</Target>
+    <Left>baseline/Avalonia/lib/net10.0/Avalonia.Controls.dll</Left>
+    <Right>current/Avalonia/lib/net10.0/Avalonia.Controls.dll</Right>
+  </Suppression>
+  <Suppression>
+    <DiagnosticId>CP0002</DiagnosticId>
+    <Target>F:Avalonia.Controls.TextBox.LetterSpacingProperty</Target>
+    <Left>baseline/Avalonia/lib/net10.0/Avalonia.Controls.dll</Left>
+    <Right>current/Avalonia/lib/net10.0/Avalonia.Controls.dll</Right>
+  </Suppression>
   <Suppression>
     <DiagnosticId>CP0002</DiagnosticId>
     <Target>F:Avalonia.Media.Fonts.FontCollectionBase._glyphTypefaceCache</Target>
@@ -103,6 +115,18 @@
     <Left>baseline/Avalonia/lib/net8.0/Avalonia.Base.dll</Left>
     <Right>current/Avalonia/lib/net8.0/Avalonia.Base.dll</Right>
   </Suppression>
+  <Suppression>
+    <DiagnosticId>CP0002</DiagnosticId>
+    <Target>F:Avalonia.Controls.TextBlock.LetterSpacingProperty</Target>
+    <Left>baseline/Avalonia/lib/net8.0/Avalonia.Controls.dll</Left>
+    <Right>current/Avalonia/lib/net8.0/Avalonia.Controls.dll</Right>
+  </Suppression>
+  <Suppression>
+    <DiagnosticId>CP0002</DiagnosticId>
+    <Target>F:Avalonia.Controls.TextBox.LetterSpacingProperty</Target>
+    <Left>baseline/Avalonia/lib/net8.0/Avalonia.Controls.dll</Left>
+    <Right>current/Avalonia/lib/net8.0/Avalonia.Controls.dll</Right>
+  </Suppression>
   <Suppression>
     <DiagnosticId>CP0002</DiagnosticId>
     <Target>M:Avalonia.Dialogs.Internal.ManagedFileChooserFilterViewModel.#ctor(Avalonia.Platform.Storage.FilePickerFileType)</Target>

samples/ControlCatalog/Pages/ButtonsPage.xaml

@@ -91,6 +91,8 @@
             </Button.Styles>
           </Button>
           <Button Classes="accent">Accent</Button>
+          <Button LetterSpacing="0">Normal Spacing</Button>
+          <Button LetterSpacing="2.0">Loose Spacing</Button>
         </StackPanel>
 
         <StackPanel Orientation="Vertical"
@@ -105,6 +107,8 @@
                   IsEnabled="False">Disabled</Button>
           <Button BorderBrush="{DynamicResource SystemAccentColor}"
                   KeyboardNavigation.IsTabStop="False">IsTabStop=False</Button>
+          <Button LetterSpacing="-0.5">Tight Spacing</Button>
+          <Button LetterSpacing="4.0">E X T R A   L O O S E</Button>
         </StackPanel>
       </StackPanel>
     </Border>

src/Avalonia.Controls/Documents/TextElement.cs

@@ -76,6 +76,21 @@ namespace Avalonia.Controls.Documents
                 Brushes.Black,
                 inherits: true);
 
+        /// <summary>
+        /// Defines the <see cref="LetterSpacing"/> property.
+        /// </summary>
+        /// <remarks>
+        /// This is an inherited attached property that defines letter spacing for text.
+        /// Letter spacing is specified in pixels. Default value is 0 (normal spacing).
+        /// Positive values increase spacing between characters.
+        /// Negative values decrease spacing between characters.
+        /// </remarks>
+        public static readonly AttachedProperty<double> LetterSpacingProperty =
+            AvaloniaProperty.RegisterAttached<TextElement, Control, double>(
+                name: nameof(LetterSpacing),
+                defaultValue: 0.0,
+                inherits: true);
+
         private IInlineHost? _inlineHost;
 
         /// <summary>
@@ -150,6 +165,15 @@ namespace Avalonia.Controls.Documents
             set => SetValue(ForegroundProperty, value);
         }
         
+        /// <summary>
+        /// Gets or sets the letter spacing.
+        /// </summary>
+        public double LetterSpacing
+        {
+            get => GetValue(LetterSpacingProperty);
+            set => SetValue(LetterSpacingProperty, value);
+        }
+
         /// <summary>
         /// Gets the value of the attached <see cref="FontFamilyProperty"/> on a control.
         /// </summary>
@@ -190,6 +214,26 @@ namespace Avalonia.Controls.Documents
             control.SetValue(FontFeaturesProperty, value);
         }
         
+        /// <summary>
+        /// Gets the value of the attached <see cref="LetterSpacingProperty"/> on a control.
+        /// </summary>
+        /// <param name="control">The control.</param>
+        /// <returns>The letter spacing applied to the control.</returns>
+        public static double GetLetterSpacing(Control control)
+        {
+            return control.GetValue(LetterSpacingProperty);
+        }
+
+        /// <summary>
+        /// Sets the value of the attached <see cref="LetterSpacingProperty"/> on a control.
+        /// </summary>
+        /// <param name="control">The control.</param>
+        /// <param name="value">The letter spacing to apply.</param>
+        public static void SetLetterSpacing(Control control, double value)
+        {
+            control.SetValue(LetterSpacingProperty, value);
+        }
+        
         /// <summary>
         /// Gets the value of the attached <see cref="FontSizeProperty"/> on a control.
         /// </summary>

src/Avalonia.Controls/Presenters/ContentPresenter.cs

@@ -118,6 +118,12 @@ namespace Avalonia.Controls.Presenters
         public static readonly StyledProperty<double> LineHeightProperty =
             TextBlock.LineHeightProperty.AddOwner<ContentPresenter>();
 
+        /// <summary>
+        /// Defines the <see cref="LetterSpacing"/> property
+        /// </summary>
+        public static readonly StyledProperty<double> LetterSpacingProperty =
+            TextElement.LetterSpacingProperty.AddOwner<ContentPresenter>();
+
         /// <summary>
         /// Defines the <see cref="MaxLines"/> property
         /// </summary>
@@ -327,6 +333,15 @@ namespace Avalonia.Controls.Presenters
             set => SetValue(LineHeightProperty, value);
         }
 
+        /// <summary>
+        /// Gets or sets the letter spacing
+        /// </summary>
+        public double LetterSpacing
+        {
+            get => GetValue(LetterSpacingProperty);
+            set => SetValue(LetterSpacingProperty, value);
+        }
+
         /// <summary>
         /// Gets or sets the max lines
         /// </summary>

src/Avalonia.Controls/Presenters/TextPresenter.cs

@@ -87,7 +87,7 @@ namespace Avalonia.Controls.Presenters
         /// Defines the <see cref="LetterSpacing"/> property.
         /// </summary>
         public static readonly StyledProperty<double> LetterSpacingProperty =
-            TextBlock.LetterSpacingProperty.AddOwner<TextPresenter>();
+            TextElement.LetterSpacingProperty.AddOwner<TextPresenter>();
 
         /// <summary>
         /// Defines the <see cref="Background"/> property.

src/Avalonia.Controls/Primitives/TemplatedControl.cs

@@ -88,6 +88,12 @@ namespace Avalonia.Controls.Primitives
         public static readonly StyledProperty<IBrush?> ForegroundProperty =
             TextElement.ForegroundProperty.AddOwner<TemplatedControl>();
 
+        /// <summary>
+        /// Defines the <see cref="LetterSpacing"/> property.
+        /// </summary>
+        public static readonly StyledProperty<double> LetterSpacingProperty =
+            TextElement.LetterSpacingProperty.AddOwner<TemplatedControl>();
+
         /// <summary>
         /// Defines the <see cref="Padding"/> property.
         /// </summary>
@@ -242,6 +248,15 @@ namespace Avalonia.Controls.Primitives
             set => SetValue(ForegroundProperty, value);
         }
 
+        /// <summary>
+        /// Gets or sets the letter spacing for the control's text content.
+        /// </summary>
+        public double LetterSpacing
+        {
+            get => GetValue(LetterSpacingProperty);
+            set => SetValue(LetterSpacingProperty, value);
+        }
+
         /// <summary>
         /// Gets or sets the padding placed between the border of the control and its content.
         /// </summary>

src/Avalonia.Controls/TextBlock.cs

@@ -97,11 +97,13 @@ namespace Avalonia.Controls
         /// <summary>
         /// Defines the <see cref="LetterSpacing"/> property.
         /// </summary>
-        public static readonly AttachedProperty<double> LetterSpacingProperty =
+        /// <remarks>
-            AvaloniaProperty.RegisterAttached<TextBlock, Control, double>(
+        /// This property uses <see cref="AvaloniaProperty.AddOwner{TOwner}(AvaloniaProperty)"/> to share the same
-                nameof(LetterSpacing),
+        /// definition as <see cref="TextElement.LetterSpacingProperty"/>, ensuring consistent behavior across text
-                0,
+        /// elements and templated controls.
-                inherits: true);
+        /// </remarks>
+        public static readonly StyledProperty<double> LetterSpacingProperty =
+            TextElement.LetterSpacingProperty.AddOwner<TextBlock>();
 
         /// <summary>
         /// Defines the <see cref="MaxLines"/> property.

src/Avalonia.Controls/TextBox.cs

@@ -180,12 +180,6 @@ namespace Avalonia.Controls
         public static readonly StyledProperty<double> LineHeightProperty =
             TextBlock.LineHeightProperty.AddOwner<TextBox>(new(defaultValue: double.NaN));
 
-        /// <summary>
-        /// Defines see <see cref="TextBlock.LetterSpacing"/> property.
-        /// </summary>
-        public static readonly StyledProperty<double> LetterSpacingProperty =
-            TextBlock.LetterSpacingProperty.AddOwner<TextBox>();
-
         /// <summary>
         /// Defines the <see cref="Watermark"/> property
         /// </summary>
@@ -570,15 +564,6 @@ namespace Avalonia.Controls
             set => SetValue(MinLinesProperty, value);
         }
 
-        /// <summary>
-        /// Gets or sets the spacing between characters
-        /// </summary>
-        public double LetterSpacing
-        {
-            get => GetValue(LetterSpacingProperty);
-            set => SetValue(LetterSpacingProperty, value);
-        }
-
         /// <summary>
         /// Gets or sets the line height.
         /// </summary>

src/Avalonia.Themes.Fluent/Controls/Label.xaml

@@ -15,7 +15,7 @@
                           Content="{TemplateBinding Content}"
                           Padding="{TemplateBinding Padding}"
                           VerticalContentAlignment="{TemplateBinding VerticalContentAlignment}"
-                          HorizontalContentAlignment="{TemplateBinding HorizontalContentAlignment}"/>
+                          HorizontalContentAlignment="{TemplateBinding HorizontalContentAlignment}" />
       </ControlTemplate>
     </Setter>
 

src/Avalonia.Themes.Fluent/Controls/ToolTip.xaml

@@ -46,6 +46,7 @@
                             MaxWidth="{TemplateBinding MaxWidth}"
                             Content="{TemplateBinding Content}"
                             ContentTemplate="{TemplateBinding ContentTemplate}"
+                            TextBlock.LetterSpacing="{TemplateBinding LetterSpacing}"
                             TextBlock.TextWrapping="Wrap"/>
         </Border>
       </ControlTemplate>

tests/Avalonia.Controls.UnitTests/ButtonTests.cs

@@ -619,6 +619,140 @@ namespace Avalonia.Controls.UnitTests
 
 
 
+        [Fact]
+        public void Button_LetterSpacing_Default_Value_Is_Zero()
+        {
+            var button = new Button();
+            Assert.Equal(0, button.LetterSpacing);
+        }
+
+        [Fact]
+        public void Button_LetterSpacing_Can_Be_Set_And_Retrieved()
+        {
+            var button = new Button { LetterSpacing = 2.5 };
+            Assert.Equal(2.5, button.LetterSpacing);
+        }
+
+        [Fact]
+        public void Button_LetterSpacing_Can_Be_Set_To_Negative_Value()
+        {
+            var button = new Button { LetterSpacing = -1.5 };
+            Assert.Equal(-1.5, button.LetterSpacing);
+        }
+
+        [Fact]
+        public void Button_LetterSpacing_Can_Be_Set_To_Zero()
+        {
+            var button = new Button { LetterSpacing = 5.0 };
+            button.LetterSpacing = 0;
+            Assert.Equal(0, button.LetterSpacing);
+        }
+
+        [Fact]
+        public void Button_LetterSpacing_Propagates_To_ContentPresenter()
+        {
+            using (UnitTestApplication.Start(TestServices.StyledWindow))
+            {
+                var button = new Button
+                {
+                    Content = "Test",
+                    LetterSpacing = 3.0
+                };
+                var root = new TestRoot { Child = button };
+
+                button.ApplyTemplate();
+
+                var presenter = button.Presenter;
+                Assert.NotNull(presenter);
+                Assert.Equal(3.0, presenter.LetterSpacing);
+            }
+        }
+
+        [Fact]
+        public void Button_LetterSpacing_Updates_ContentPresenter_When_Changed()
+        {
+            using (UnitTestApplication.Start(TestServices.StyledWindow))
+            {
+                var button = new Button
+                {
+                    Content = "Test",
+                    LetterSpacing = 1.0
+                };
+                var root = new TestRoot { Child = button };
+
+                button.ApplyTemplate();
+                var presenter = button.Presenter;
+
+                button.LetterSpacing = 5.0;
+
+                Assert.Equal(5.0, presenter.LetterSpacing);
+            }
+        }
+
+        [Fact]
+        public void Button_LetterSpacing_Works_With_Large_Values()
+        {
+            var button = new Button { LetterSpacing = 100.0 };
+            Assert.Equal(100.0, button.LetterSpacing);
+        }
+
+        [Fact]
+        public void Button_LetterSpacing_Property_Inherits_Through_Visual_Tree()
+        {
+            using (UnitTestApplication.Start(TestServices.StyledWindow))
+            {
+                var button = new Button
+                {
+                    Content = "Test",
+                    LetterSpacing = 2.0
+                };
+                var root = new TestRoot { Child = button };
+
+                button.ApplyTemplate();
+                button.Presenter?.UpdateChild();
+
+                // Verify the property value is accessible on the presenter
+                var presenter = button.Presenter;
+                Assert.NotNull(presenter);
+                Assert.Equal(2.0, presenter.LetterSpacing);
+            }
+        }
+
+        [Fact]
+        public void Button_LetterSpacing_Affects_TextBlock_Child_In_ContentPresenter()
+        {
+            using (UnitTestApplication.Start(TestServices.MockPlatformRenderInterface))
+            {
+                var button = new Button
+                {
+                    Content = "Test Text",
+                    LetterSpacing = 3.5
+                };
+                var root = new TestRoot { Child = button };
+
+                button.ApplyTemplate();
+                button.Presenter?.UpdateChild();
+
+                // Find the TextBlock that was created by ContentPresenter
+                var presenter = button.Presenter;
+                Assert.NotNull(presenter);
+
+                var textBlock = presenter.Child as TextBlock;
+                Assert.NotNull(textBlock);
+
+                // Verify LetterSpacing inherited to the TextBlock
+                Assert.Equal(3.5, textBlock.LetterSpacing);
+
+                // Force a measure to create TextLayout
+                textBlock.Measure(new Size(double.PositiveInfinity, double.PositiveInfinity));
+
+                // Verify the TextLayout actually has the LetterSpacing value
+                var textLayout = textBlock.TextLayout;
+                Assert.NotNull(textLayout);
+                Assert.Equal(3.5, textLayout.LetterSpacing);
+            }
+        }
+
         private class TestTopLevel : TopLevel
         {
             private readonly ILayoutManager _layoutManager;

tests/Avalonia.Controls.UnitTests/CheckBoxTests.cs

@@ -0,0 +1,37 @@
+using Avalonia.UnitTests;
+using Xunit;
+
+namespace Avalonia.Controls.UnitTests
+{
+    public class CheckBoxTests : ScopedTestBase
+    {
+        [Fact]
+        public void CheckBox_LetterSpacing_Default_Value_Is_Zero()
+        {
+            var checkBox = new CheckBox();
+            Assert.Equal(0, checkBox.LetterSpacing);
+        }
+
+        [Fact]
+        public void CheckBox_LetterSpacing_Can_Be_Set_And_Retrieved()
+        {
+            var checkBox = new CheckBox { LetterSpacing = 2.5 };
+            Assert.Equal(2.5, checkBox.LetterSpacing);
+        }
+
+        [Fact]
+        public void CheckBox_LetterSpacing_Inherits_From_TemplatedControl()
+        {
+            var checkBox = new CheckBox { LetterSpacing = 3.0 };
+            // LetterSpacing is inherited from TemplatedControl
+            Assert.Equal(3.0, checkBox.LetterSpacing);
+        }
+
+        [Fact]
+        public void CheckBox_LetterSpacing_Can_Be_Negative()
+        {
+            var checkBox = new CheckBox { LetterSpacing = -1.5 };
+            Assert.Equal(-1.5, checkBox.LetterSpacing);
+        }
+    }
+}

tests/Avalonia.Controls.UnitTests/LabelTests.cs

@@ -0,0 +1,37 @@
+using Avalonia.UnitTests;
+using Xunit;
+
+namespace Avalonia.Controls.UnitTests
+{
+    public class LabelTests : ScopedTestBase
+    {
+        [Fact]
+        public void Label_LetterSpacing_Default_Value_Is_Zero()
+        {
+            var label = new Label();
+            Assert.Equal(0, label.LetterSpacing);
+        }
+
+        [Fact]
+        public void Label_LetterSpacing_Can_Be_Set_And_Retrieved()
+        {
+            var label = new Label { LetterSpacing = 2.5 };
+            Assert.Equal(2.5, label.LetterSpacing);
+        }
+
+        [Fact]
+        public void Label_LetterSpacing_Inherits_From_TemplatedControl()
+        {
+            var label = new Label { LetterSpacing = 3.0 };
+            // LetterSpacing is inherited from TemplatedControl
+            Assert.Equal(3.0, label.LetterSpacing);
+        }
+
+        [Fact]
+        public void Label_LetterSpacing_Can_Be_Negative()
+        {
+            var label = new Label { LetterSpacing = -1.5 };
+            Assert.Equal(-1.5, label.LetterSpacing);
+        }
+    }
+}

tests/Avalonia.Controls.UnitTests/Presenters/ContentPresenterTests_Standalone.cs

@@ -12,6 +12,7 @@ using Xunit;
 using Avalonia.Rendering;
 using Avalonia.Media;
 using Avalonia.Data;
+using Avalonia.Controls.Documents;
 
 namespace Avalonia.Controls.UnitTests.Presenters
 {
@@ -333,5 +334,75 @@ namespace Avalonia.Controls.UnitTests.Presenters
 
             Assert.NotNull(target.Child);
         }
+
+        [Fact]
+        public void ContentPresenter_LetterSpacing_Default_Value_Is_Zero()
+        {
+            var presenter = new ContentPresenter();
+            Assert.Equal(0, presenter.LetterSpacing);
+        }
+
+        [Fact]
+        public void ContentPresenter_LetterSpacing_Can_Be_Set_And_Retrieved()
+        {
+            var presenter = new ContentPresenter { LetterSpacing = 3.5 };
+            Assert.Equal(3.5, presenter.LetterSpacing);
+        }
+
+        [Fact]
+        public void ContentPresenter_LetterSpacing_Can_Be_Negative()
+        {
+            var presenter = new ContentPresenter { LetterSpacing = -2.0 };
+            Assert.Equal(-2.0, presenter.LetterSpacing);
+        }
+
+        [Fact]
+        public void ContentPresenter_LetterSpacing_Propagates_To_TextBlock_Child()
+        {
+            using (UnitTestApplication.Start(TestServices.MockPlatformRenderInterface))
+            {
+                var presenter = new ContentPresenter
+                {
+                    Content = "Test Content",
+                    LetterSpacing = 4.0
+                };
+                var root = new TestRoot { Child = presenter };
+
+                presenter.UpdateChild();
+
+                var textBlock = presenter.Child as TextBlock;
+                Assert.NotNull(textBlock);
+                Assert.Equal(4.0, textBlock.LetterSpacing);
+            }
+        }
+
+        [Fact]
+        public void ContentPresenter_LetterSpacing_Updates_TextBlock_When_Changed()
+        {
+            using (UnitTestApplication.Start(TestServices.MockPlatformRenderInterface))
+            {
+                var presenter = new ContentPresenter
+                {
+                    Content = "Test Content",
+                    LetterSpacing = 1.0
+                };
+                var root = new TestRoot { Child = presenter };
+
+                presenter.UpdateChild();
+                var textBlock = presenter.Child as TextBlock;
+
+                presenter.LetterSpacing = 6.0;
+
+                Assert.NotNull(textBlock);
+                Assert.Equal(6.0, textBlock.LetterSpacing);
+            }
+        }
+
+        [Fact]
+        public void ContentPresenter_LetterSpacing_Property_Inherits_From_TextBlock()
+        {
+            // Verify that ContentPresenter's LetterSpacing uses the TextElement letter spacing definition
+            Assert.Same(TextElement.LetterSpacingProperty, ContentPresenter.LetterSpacingProperty);
+        }
     }
 }

tests/Avalonia.Controls.UnitTests/Primitives/TemplatedControlTests.cs

@@ -11,6 +11,7 @@ using Avalonia.UnitTests;
 using Avalonia.VisualTree;
 using Xunit;
 using Avalonia.Media;
+using Avalonia.Controls.Documents;
 
 namespace Avalonia.Controls.UnitTests.Primitives
 {
@@ -597,5 +598,52 @@ namespace Avalonia.Controls.UnitTests.Primitives
 
             return result;
         }
+
+        [Fact]
+        public void TemplatedControl_LetterSpacing_Default_Value_Is_Zero()
+        {
+            var target = new TestTemplatedControl();
+            Assert.Equal(0, target.LetterSpacing);
+        }
+
+        [Fact]
+        public void TemplatedControl_LetterSpacing_Uses_TextElement_Property()
+        {
+            Assert.Same(TextElement.LetterSpacingProperty, TemplatedControl.LetterSpacingProperty);
+        }
+
+        [Fact]
+        public void TemplatedControl_LetterSpacing_Can_Be_Set_And_Retrieved()
+        {
+            var target = new TestTemplatedControl { LetterSpacing = 2.5 };
+            Assert.Equal(2.5, target.LetterSpacing);
+        }
+
+        [Fact]
+        public void TemplatedControl_LetterSpacing_Can_Be_Negative()
+        {
+            var target = new TestTemplatedControl { LetterSpacing = -1.5 };
+            Assert.Equal(-1.5, target.LetterSpacing);
+        }
+
+        [Fact]
+        public void TemplatedControl_LetterSpacing_Inherits_To_ContentPresenter()
+        {
+            using (UnitTestApplication.Start(TestServices.StyledWindow))
+            {
+                var target = new ContentControl
+                {
+                    LetterSpacing = 3.0,
+                    Content = "Test",
+                };
+                var root = new TestRoot { Child = target };
+
+                target.ApplyTemplate();
+
+                var presenter = target.Presenter;
+                Assert.NotNull(presenter);
+                Assert.Equal(3.0, presenter.LetterSpacing);
+            }
+        }
     }
 }

tests/Avalonia.Controls.UnitTests/RadioButtonTests.cs

@@ -106,8 +106,28 @@ namespace Avalonia.Controls.UnitTests
             Assert.False(radioButton2.IsChecked);
             Assert.True(radioButton3.IsChecked);
             Assert.False(radioButton4.IsChecked);
+        }
+
+        [Fact]
+        public void RadioButton_LetterSpacing_Default_Value_Is_Zero()
+        {
+            var radioButton = new RadioButton();
+            Assert.Equal(0, radioButton.LetterSpacing);
+        }
 
+        [Fact]
+        public void RadioButton_LetterSpacing_Can_Be_Set_And_Retrieved()
+        {
+            var radioButton = new RadioButton { LetterSpacing = 2.5 };
+            Assert.Equal(2.5, radioButton.LetterSpacing);
+        }
 
+        [Fact]
+        public void RadioButton_LetterSpacing_Inherits_From_TemplatedControl()
+        {
+            var radioButton = new RadioButton { LetterSpacing = 3.0 };
+            // LetterSpacing is inherited from TemplatedControl
+            Assert.Equal(3.0, radioButton.LetterSpacing);
         }
     }
 }

tests/Avalonia.Controls.UnitTests/TextBlockTests.cs

@@ -28,6 +28,12 @@ namespace Avalonia.Controls.UnitTests
             Assert.Equal(null, textBlock.Text);
         }
 
+        [Fact]
+        public void LetterSpacing_Property_Uses_TextElement_Definition()
+        {
+            Assert.Same(TextElement.LetterSpacingProperty, TextBlock.LetterSpacingProperty);
+        }
+
         [Fact]
         public void Calling_Measure_Should_Update_TextLayout()
         {