Implement VerticalContentAlignment for DatePicker and TimePicker (#21464)

* Add VerticalContentAlignment property to DatePicker and TimePicker

This change registers VerticalContentAlignmentProperty as a
StyledProperty using ContentControl's property as the owner for both
DatePicker and TimePicker controls to maintain framework consistency.

Basic xUnit v3 unit tests are introduced to validate property
round-trips (Top, Center, Bottom, Stretch) and to ensure the default
value correctly resolves to Stretch. This establishes the necessary
C# infrastructure before modifying the control templates.

Part of #21211: implment fluent theme layout

* Fix VerticalContentAlignment bindings and integration tests

- Bind internal grids to VerticalContentAlignment in Fluent and Simple
  themes for DatePicker and TimePicker controls.
- Add comprehensive integration tests using mocked NameScopes in
  DatePickerTests and TimePickerTests to verify visual propagation.
- Validate visual layout correctness using the ControlCatalog app.

Closes AvaloniaUI#21211

* Remove tautological tests

---------

Co-authored-by: Martim Claudino <martimffclaudino@tecnico.ulisboa.pt>
Co-authored-by: Julien Lebosquain <julien@lebosquain.net>

src/Avalonia.Controls/DateTimePickers/DatePicker.cs

@@ -75,6 +75,21 @@ namespace Avalonia.Controls
         public static readonly StyledProperty<bool> YearVisibleProperty =
             AvaloniaProperty.Register<DatePicker, bool>(nameof(YearVisible), true);
 
+        /// <summary>
+        /// Defines the <see cref="VerticalContentAlignment"/> property.
+        /// </summary>
+        public static readonly StyledProperty<VerticalAlignment> VerticalContentAlignmentProperty =
+            ContentControl.VerticalContentAlignmentProperty.AddOwner<DatePicker>();
+
+        /// <summary>
+        /// Gets or sets the vertical alignment of the content within the control.
+        /// </summary>
+        public VerticalAlignment VerticalContentAlignment
+        {
+            get => GetValue(VerticalContentAlignmentProperty);
+            set => SetValue(VerticalContentAlignmentProperty, value);
+        }
+
         /// <summary>
         /// Defines the <see cref="SelectedDate"/> Property
         /// </summary>

src/Avalonia.Controls/DateTimePickers/TimePicker.cs

@@ -50,6 +50,20 @@ namespace Avalonia.Controls
         public static readonly StyledProperty<string> ClockIdentifierProperty =
            AvaloniaProperty.Register<TimePicker, string>(nameof(ClockIdentifier), "12HourClock", coerce: CoerceClockIdentifier);
         
+        /// <summary>
+        /// Defines the <see cref="VerticalContentAlignment"/> property.
+        /// </summary>
+        public static readonly StyledProperty<VerticalAlignment> VerticalContentAlignmentProperty =
+            ContentControl.VerticalContentAlignmentProperty.AddOwner<TimePicker>();
+
+        /// <summary>
+        /// Gets or sets the vertical alignment of the content within the control.
+        /// </summary>
+        public VerticalAlignment VerticalContentAlignment
+        {
+            get => GetValue(VerticalContentAlignmentProperty);
+            set => SetValue(VerticalContentAlignmentProperty, value);
+        }
         /// <summary>
         /// Defines the <see cref="UseSeconds"/> property
         /// </summary>

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

@@ -98,7 +98,7 @@
                     HorizontalAlignment="Stretch"
                     VerticalAlignment="Stretch"
                     TemplatedControl.IsTemplateFocusTarget="True">
-              <Grid Name="PART_ButtonContentGrid" ColumnDefinitions="78*,Auto,132*,Auto,78*">
+              <Grid Name="PART_ButtonContentGrid" ColumnDefinitions="78*,Auto,132*,Auto,78*" VerticalAlignment="{TemplateBinding VerticalContentAlignment}">
                 <TextBlock Name="PART_DayTextBlock" Text="{DynamicResource StringDatePickerDayText}" HorizontalAlignment="Center"
                            Padding="{DynamicResource DatePickerHostPadding}" />
                 <TextBlock Name="PART_MonthTextBlock" Text="{DynamicResource StringDatePickerMonthText}" TextAlignment="Left"

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

@@ -97,7 +97,7 @@
                     HorizontalAlignment="Stretch"
                     VerticalAlignment="Stretch">
 
-              <Grid Name="PART_FlyoutButtonContentGrid">
+              <Grid Name="PART_FlyoutButtonContentGrid" VerticalAlignment="{TemplateBinding VerticalContentAlignment}">
                 <!--Ignore col defs here, set in code-->
                 <Border x:Name="PART_FirstPickerHost"
                         Grid.Column="0"

src/Avalonia.Themes.Simple/Controls/DatePicker.xaml

@@ -106,7 +106,8 @@
                     TemplatedControl.IsTemplateFocusTarget="True"
                     Theme="{StaticResource SimpleDatePickerFlyoutButton}">
               <Grid Name="PART_ButtonContentGrid"
-                    ColumnDefinitions="78*,Auto,132*,Auto,78*">
+                    ColumnDefinitions="78*,Auto,132*,Auto,78*"
+                    VerticalAlignment="{TemplateBinding VerticalContentAlignment}">
                 <TextBlock Name="PART_DayTextBlock"
                            Padding="{DynamicResource DatePickerHostPadding}"
                            HorizontalAlignment="Center"

src/Avalonia.Themes.Simple/Controls/TimePicker.xaml

@@ -103,7 +103,7 @@
                     IsEnabled="{TemplateBinding IsEnabled}"
                     Theme="{StaticResource SimpleTimePickerFlyoutButton}">
 
-              <Grid Name="PART_FlyoutButtonContentGrid">
+              <Grid Name="PART_FlyoutButtonContentGrid" VerticalAlignment="{TemplateBinding VerticalContentAlignment}">
                 <!--  Ignore col defs here, set in code  -->
                 <Border x:Name="PART_FirstPickerHost"
                         Grid.Column="0"

tests/Avalonia.Controls.UnitTests/DatePickerTests.cs

@@ -13,6 +13,7 @@ using Avalonia.Platform;
 using Avalonia.Threading;
 using Avalonia.UnitTests;
 using Avalonia.VisualTree;
+using Avalonia.Layout;
 using Moq;
 using Xunit;
 
@@ -371,6 +372,24 @@ namespace Avalonia.Controls.UnitTests
             });
         }
 
+        [Theory]
+        [InlineData(Avalonia.Layout.VerticalAlignment.Top)]
+        [InlineData(Avalonia.Layout.VerticalAlignment.Center)]
+        [InlineData(Avalonia.Layout.VerticalAlignment.Bottom)]
+        [InlineData(Avalonia.Layout.VerticalAlignment.Stretch)]
+        public void VerticalContentAlignment_RoundTrips(Avalonia.Layout.VerticalAlignment value)
+        {
+            var datePicker = new DatePicker { VerticalContentAlignment = value };
+            Assert.Equal(value, datePicker.VerticalContentAlignment);
+        }
+
+        [Fact]
+        public void VerticalContentAlignment_Default_Is_Stretch()
+        {
+            var datePicker = new DatePicker();
+            Assert.Equal(Avalonia.Layout.VerticalAlignment.Stretch, datePicker.VerticalContentAlignment);
+        }
+
         private static IControlTemplate CreatePickerTemplate()
         {
             return new FuncControlTemplate((_, scope) =>

tests/Avalonia.Controls.UnitTests/TimePickerTests.cs

@@ -11,6 +11,7 @@ using Avalonia.Platform;
 using Avalonia.Threading;
 using Avalonia.UnitTests;
 using Avalonia.VisualTree;
+using Avalonia.Layout;
 using Moq;
 using Xunit;
 
@@ -466,5 +467,23 @@ namespace Avalonia.Controls.UnitTests
                 return contentPanel;
             });
         }
+
+        [Theory]
+        [InlineData(Avalonia.Layout.VerticalAlignment.Top)]
+        [InlineData(Avalonia.Layout.VerticalAlignment.Center)]
+        [InlineData(Avalonia.Layout.VerticalAlignment.Bottom)]
+        [InlineData(Avalonia.Layout.VerticalAlignment.Stretch)]
+        public void VerticalContentAlignment_RoundTrips(Avalonia.Layout.VerticalAlignment value)
+        {
+            var timePicker = new TimePicker { VerticalContentAlignment = value };
+            Assert.Equal(value, timePicker.VerticalContentAlignment);
+        }
+
+        [Fact]
+        public void VerticalContentAlignment_Default_Is_Stretch()
+        {
+            var timePicker = new TimePicker();
+            Assert.Equal(Avalonia.Layout.VerticalAlignment.Stretch, timePicker.VerticalContentAlignment);
+        }
     }
 }