Add HorizontalContentAlignment and VerticalContentAlignment to ComboBox default and fluent style

src/Avalonia.Controls/ComboBox.cs

@@ -7,6 +7,7 @@ using Avalonia.Controls.Shapes;
 using Avalonia.Controls.Templates;
 using Avalonia.Input;
 using Avalonia.Interactivity;
+using Avalonia.Layout;
 using Avalonia.LogicalTree;
 using Avalonia.Media;
 using Avalonia.VisualTree;
@@ -63,6 +64,18 @@ namespace Avalonia.Controls
         public static readonly StyledProperty<IBrush> PlaceholderForegroundProperty =
             AvaloniaProperty.Register<ComboBox, IBrush>(nameof(PlaceholderForeground));
 
+        /// <summary>
+        /// Defines the <see cref="HorizontalContentAlignment"/> property.
+        /// </summary>
+        public static readonly StyledProperty<HorizontalAlignment> HorizontalContentAlignmentProperty =
+            ContentControl.HorizontalContentAlignmentProperty.AddOwner<ComboBox>();
+
+        /// <summary>
+        /// Defines the <see cref="VerticalContentAlignment"/> property.
+        /// </summary>
+        public static readonly StyledProperty<VerticalAlignment> VerticalContentAlignmentProperty =
+            ContentControl.VerticalContentAlignmentProperty.AddOwner<ComboBox>();
+
         private bool _isDropDownOpen;
         private Popup _popup;
         private object _selectionBoxItem;
@@ -133,6 +146,24 @@ namespace Avalonia.Controls
             set { SetValue(VirtualizationModeProperty, value); }
         }
 
+        /// <summary>
+        /// Gets or sets the horizontal alignment of the content within the control.
+        /// </summary>
+        public HorizontalAlignment HorizontalContentAlignment
+        {
+            get { return GetValue(HorizontalContentAlignmentProperty); }
+            set { SetValue(HorizontalContentAlignmentProperty, value); }
+        }
+
+        /// <summary>
+        /// Gets or sets the vertical alignment of the content within the control.
+        /// </summary>
+        public VerticalAlignment VerticalContentAlignment
+        {
+            get { return GetValue(VerticalContentAlignmentProperty); }
+            set { SetValue(VerticalContentAlignmentProperty, value); }
+        }
+
         /// <inheritdoc/>
         protected override IItemContainerGenerator CreateItemContainerGenerator()
         {

src/Avalonia.Themes.Default/ComboBox.xaml

@@ -1,10 +1,31 @@
 <Styles xmlns="https://github.com/avaloniaui">
+  <Design.PreviewWith>
+    <Border Padding="20">
+      <StackPanel Spacing="10">
+        <ComboBox PlaceholderText="Select an item">
+          <ComboBoxItem>Item 1</ComboBoxItem>
+          <ComboBoxItem>Item 2</ComboBoxItem>
+        </ComboBox>
+        <ComboBox IsEnabled="False"
+                  Width="200"
+                  SelectedIndex="1"
+                  HorizontalContentAlignment="Center">
+          <ComboBoxItem>Item 1</ComboBoxItem>
+          <ComboBoxItem>Item 2</ComboBoxItem>
+        </ComboBox>
+      </StackPanel>
+    </Border>
+  </Design.PreviewWith>
   <Style Selector="ComboBox">
-    <Setter Property="Background" Value="Transparent"/>
-    <Setter Property="BorderBrush" Value="{DynamicResource ThemeBorderMidBrush}"/>
-    <Setter Property="BorderThickness" Value="{DynamicResource ThemeBorderThickness}"/>
-    <Setter Property="Padding" Value="4"/>
-    <Setter Property="MinHeight" Value="20"/>
+    <Setter Property="Background" Value="Transparent" />
+    <Setter Property="BorderBrush" Value="{DynamicResource ThemeBorderMidBrush}" />
+    <Setter Property="BorderThickness" Value="{DynamicResource ThemeBorderThickness}" />
+    <Setter Property="HorizontalAlignment" Value="Left" />
+    <Setter Property="VerticalAlignment" Value="Top" />
+    <Setter Property="HorizontalContentAlignment" Value="Stretch" />
+    <Setter Property="VerticalContentAlignment" Value="Center" />
+    <Setter Property="Padding" Value="4" />
+    <Setter Property="MinHeight" Value="20" />
     <Setter Property="Template">
       <ControlTemplate>
         <Border Name="border"
@@ -15,8 +36,8 @@
             <ContentControl Content="{TemplateBinding SelectionBoxItem}"
                             ContentTemplate="{TemplateBinding ItemTemplate}"
                             Margin="{TemplateBinding Padding}"
-                            HorizontalAlignment="Left"
-                            VerticalAlignment="Center"/>
+                            VerticalContentAlignment="{TemplateBinding VerticalContentAlignment}"
+                            HorizontalContentAlignment="{TemplateBinding HorizontalContentAlignment}" />
             <ToggleButton Name="toggle"
                           BorderThickness="0"
                           Background="Transparent"
@@ -30,7 +51,7 @@
                     Stretch="Uniform"
                     HorizontalAlignment="Center"
                     VerticalAlignment="Center"
-                    Data="F1 M 301.14,-189.041L 311.57,-189.041L 306.355,-182.942L 301.14,-189.041 Z"/>
+                    Data="F1 M 301.14,-189.041L 311.57,-189.041L 306.355,-182.942L 301.14,-189.041 Z" />
             </ToggleButton>
             <Popup Name="PART_Popup"
                    IsOpen="{TemplateBinding IsDropDownOpen, Mode=TwoWay}"
@@ -45,8 +66,7 @@
                                   Items="{TemplateBinding Items}"
                                   ItemsPanel="{TemplateBinding ItemsPanel}"
                                   ItemTemplate="{TemplateBinding ItemTemplate}"
-                                  VirtualizationMode="{TemplateBinding VirtualizationMode}"
-                              />
+                                  VirtualizationMode="{TemplateBinding VirtualizationMode}" />
                 </ScrollViewer>
               </Border>
             </Popup>
@@ -56,7 +76,7 @@
     </Setter>
   </Style>
   <Style Selector="ComboBox:pointerover /template/ Border#border">
-    <Setter Property="BorderBrush" Value="{DynamicResource ThemeBorderHighBrush}"/>
+    <Setter Property="BorderBrush" Value="{DynamicResource ThemeBorderHighBrush}" />
   </Style>
   <Style Selector="ComboBox:disabled /template/ Border#border">
     <Setter Property="Opacity" Value="{DynamicResource ThemeDisabledOpacity}" />

src/Avalonia.Themes.Fluent/ComboBox.xaml

@@ -7,8 +7,10 @@
           <ComboBoxItem>Item 1</ComboBoxItem>
           <ComboBoxItem>Item 2</ComboBoxItem>
         </ComboBox>
-
-        <ComboBox IsEnabled="False">
+        <ComboBox IsEnabled="False"
+                  Width="200"
+                  SelectedIndex="1"
+                  HorizontalContentAlignment="Center">
           <ComboBoxItem>Item 1</ComboBoxItem>
           <ComboBoxItem>Item 2</ComboBoxItem>
         </ComboBox>
@@ -34,6 +36,8 @@
     <Setter Property="BorderThickness" Value="{DynamicResource ComboBoxBorderThemeThickness}" />
     <Setter Property="ScrollViewer.HorizontalScrollBarVisibility" Value="Disabled" />
     <Setter Property="ScrollViewer.VerticalScrollBarVisibility" Value="Auto" />
+    <Setter Property="HorizontalContentAlignment" Value="Stretch" />
+    <Setter Property="VerticalContentAlignment" Value="Center" />
     <Setter Property="HorizontalAlignment" Value="Left" />
     <Setter Property="VerticalAlignment" Value="Top" />
     <Setter Property="FontFamily" Value="{DynamicResource ContentControlThemeFontFamily}" />
@@ -71,8 +75,8 @@
           <TextBlock x:Name="PlaceholderTextBlock"
                      Grid.Row="1"
                      Grid.Column="0"
-                     HorizontalAlignment="Left"
-                     VerticalAlignment="Center"
+                     HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}"
+                     VerticalAlignment="{TemplateBinding VerticalContentAlignment}"
                      Margin="{TemplateBinding Padding}"
                      Text="{TemplateBinding PlaceholderText}"
                      IsVisible="{TemplateBinding SelectionBoxItem, Converter={x:Static ObjectConverters.IsNull}}" />
@@ -82,8 +86,8 @@
                           Grid.Row="1"
                           Grid.Column="0"
                           Margin="{TemplateBinding Padding}"
-                          HorizontalAlignment="Left"
-                          VerticalAlignment="Center" />
+                          HorizontalContentAlignment="{TemplateBinding HorizontalContentAlignment}"
+                          VerticalContentAlignment="{TemplateBinding VerticalContentAlignment}" />
 
           <Border x:Name="DropDownOverlay"
                   Grid.Row="1"