feat: WindowNotificationManager: support TopCenter and BottomCenter position (#12804)

* feat: support topcenter and bottom center.

* fix: fix typo.

src/Avalonia.Controls/Notifications/NotificationPosition.cs

@@ -9,6 +9,8 @@
         TopLeft,
         TopRight,
         BottomLeft,
-        BottomRight
+        BottomRight,
+        TopCenter,
+        BottomCenter,
     }
 }

src/Avalonia.Controls/Notifications/WindowNotificationManager.cs

@@ -15,7 +15,7 @@ namespace Avalonia.Controls.Notifications
     /// An <see cref="INotificationManager"/> that displays notifications in a <see cref="Window"/>.
     /// </summary>
     [TemplatePart("PART_Items", typeof(Panel))]
-    [PseudoClasses(":topleft", ":topright", ":bottomleft", ":bottomright")]
+    [PseudoClasses(":topleft", ":topright", ":bottomleft", ":bottomright", ":topcenter", ":bottomcenter")]
     public class WindowNotificationManager : TemplatedControl, IManagedNotificationManager
     {
         private IList? _items;
@@ -215,6 +215,8 @@ namespace Avalonia.Controls.Notifications
             PseudoClasses.Set(":topright", position == NotificationPosition.TopRight);
             PseudoClasses.Set(":bottomleft", position == NotificationPosition.BottomLeft);
             PseudoClasses.Set(":bottomright", position == NotificationPosition.BottomRight);
+            PseudoClasses.Set(":topcenter", position == NotificationPosition.TopCenter);
+            PseudoClasses.Set(":bottomcenter", position == NotificationPosition.BottomCenter);
         }
     }
 }

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

@@ -42,6 +42,11 @@
       <Setter Property="HorizontalAlignment" Value="Right" />
     </Style>
 
+    <Style Selector="^:topcenter /template/ ReversibleStackPanel#PART_Items">
+      <Setter Property="VerticalAlignment" Value="Top" />
+      <Setter Property="HorizontalAlignment" Value="Center" />
+    </Style>
+
     <Style Selector="^:bottomleft /template/ ReversibleStackPanel#PART_Items">
       <Setter Property="ReverseOrder" Value="True" />
       <Setter Property="VerticalAlignment" Value="Bottom" />
@@ -53,5 +58,11 @@
       <Setter Property="VerticalAlignment" Value="Bottom" />
       <Setter Property="HorizontalAlignment" Value="Right" />
     </Style>
+
+    <Style Selector="^:bottomcenter /template/ ReversibleStackPanel#PART_Items">
+      <Setter Property="ReverseOrder" Value="True" />
+      <Setter Property="VerticalAlignment" Value="Bottom" />
+      <Setter Property="HorizontalAlignment" Value="Center" />
+    </Style>
   </ControlTheme>
 </ResourceDictionary>

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

@@ -38,6 +38,11 @@
       <Setter Property="HorizontalAlignment" Value="Right" />
     </Style>
 
+    <Style Selector="^:topcenter /template/ ReversibleStackPanel#PART_Items">
+      <Setter Property="VerticalAlignment" Value="Top" />
+      <Setter Property="HorizontalAlignment" Value="Center" />
+    </Style>
+
     <Style Selector="^:bottomleft /template/ ReversibleStackPanel#PART_Items">
       <Setter Property="ReverseOrder" Value="True" />
       <Setter Property="VerticalAlignment" Value="Bottom" />
@@ -49,5 +54,11 @@
       <Setter Property="VerticalAlignment" Value="Bottom" />
       <Setter Property="HorizontalAlignment" Value="Right" />
     </Style>
+
+    <Style Selector="^:bottomcenter /template/ ReversibleStackPanel#PART_Items">
+      <Setter Property="ReverseOrder" Value="True" />
+      <Setter Property="VerticalAlignment" Value="Bottom" />
+      <Setter Property="HorizontalAlignment" Value="Center" />
+    </Style>
   </ControlTheme>
 </ResourceDictionary>