use a seperate control for stackpanel reversing until we have flow control available.

src/Avalonia.Controls/Notifications/ReversibleStackPanel.cs

@@ -0,0 +1,61 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using Avalonia.Layout;
+using Avalonia.Styling;
+
+namespace Avalonia.Controls
+{
+    public class ReversibleStackPanel : StackPanel
+    {
+        /// <summary>
+        /// Defines the <see cref="ReverseOrder"/> property.
+        /// </summary>
+        public static readonly StyledProperty<bool> ReverseOrderProperty =
+            AvaloniaProperty.Register<ReversibleStackPanel, bool>(nameof(ReverseOrder));        
+
+        /// <summary>
+        /// Gets or sets if the child controls will be layed out in reverse order.
+        /// </summary>
+        public bool ReverseOrder
+        {
+            get => GetValue(ReverseOrderProperty);
+            set => SetValue(ReverseOrderProperty, value);
+        }
+
+        /// <inheritdoc/>
+        protected override Size ArrangeOverride(Size finalSize)
+        {
+            var orientation = Orientation;
+            var spacing = Spacing;
+            var finalRect = new Rect(finalSize);
+            var pos = 0.0;
+
+            var children = ReverseOrder ? Children.Reverse() : Children;
+
+            foreach (Control child in children)
+            {
+                double childWidth = child.DesiredSize.Width;
+                double childHeight = child.DesiredSize.Height;
+
+                if (orientation == Orientation.Vertical)
+                {
+                    var rect = new Rect(0, pos, childWidth, childHeight)
+                        .Align(finalRect, child.HorizontalAlignment, VerticalAlignment.Top);
+                    ArrangeChild(child, rect, finalSize, orientation);
+                    pos += childHeight + spacing;
+                }
+                else
+                {
+                    var rect = new Rect(pos, 0, childWidth, childHeight)
+                        .Align(finalRect, HorizontalAlignment.Left, child.VerticalAlignment);
+                    ArrangeChild(child, rect, finalSize, orientation);
+                    pos += childWidth + spacing;
+                }
+            }
+
+            return finalSize;
+        }
+    }
+}

src/Avalonia.Controls/StackPanel.cs

@@ -25,12 +25,6 @@ namespace Avalonia.Controls
         public static readonly StyledProperty<Orientation> OrientationProperty =
             AvaloniaProperty.Register<StackPanel, Orientation>(nameof(Orientation), Orientation.Vertical);
 
-        /// <summary>
-        /// Defines the <see cref="ReverseOrder"/> property.
-        /// </summary>
-        public static readonly StyledProperty<bool> ReverseOrderProperty =
-            AvaloniaProperty.Register<StackPanel, bool>(nameof(ReverseOrder));
-
         /// <summary>
         /// Initializes static members of the <see cref="StackPanel"/> class.
         /// </summary>
@@ -58,15 +52,6 @@ namespace Avalonia.Controls
             set { SetValue(OrientationProperty, value); }
         }
 
-        /// <summary>
-        /// Gets or sets if the child controls will be layed out in reverse order.
-        /// </summary>
-        public bool ReverseOrder
-        {
-            get => GetValue(ReverseOrderProperty);
-            set => SetValue(ReverseOrderProperty, value);
-        }
-
         /// <summary>
         /// Gets the next control in the specified direction.
         /// </summary>
@@ -246,7 +231,7 @@ namespace Avalonia.Controls
             var finalRect = new Rect(finalSize);
             var pos = 0.0;
 
-            var children = ReverseOrder ? Children.Reverse() : Children;
+            var children = Children;
 
             foreach (Control child in children)
             {

src/Avalonia.Themes.Default/WindowNotificationManager.xaml

@@ -4,8 +4,8 @@
         <Setter Property="Margin" Value="0 0 8 8"/>
         <Setter Property="Template">
             <ControlTemplate>
-                <StackPanel Name="PART_Items">
-                    <StackPanel.DataTemplates>
+                <ReversibleStackPanel Name="PART_Items">
+                    <ReversibleStackPanel.DataTemplates>
                         <DataTemplate DataType="INotification">
                             <StackPanel Spacing="8" Margin="12">
                                 <TextBlock Text="{Binding Title}" FontWeight="Medium" />
@@ -15,29 +15,29 @@
                         <DataTemplate DataType="x:String">
                             <TextBlock Text="{Binding }" Margin="12" />
                         </DataTemplate>
-                    </StackPanel.DataTemplates>
-                </StackPanel>
+                    </ReversibleStackPanel.DataTemplates>
+                </ReversibleStackPanel>
             </ControlTemplate>
         </Setter>
     </Style>
 
-    <Style Selector="WindowNotificationManager:topleft /template/ StackPanel#PART_Items">
+    <Style Selector="WindowNotificationManager:topleft /template/ ReversibleStackPanel#PART_Items">
         <Setter Property="VerticalAlignment" Value="Top"/>
         <Setter Property="HorizontalAlignment" Value="Left"/>
     </Style>
 
-    <Style Selector="WindowNotificationManager:topright /template/ StackPanel#PART_Items">
+    <Style Selector="WindowNotificationManager:topright /template/ ReversibleStackPanel#PART_Items">
         <Setter Property="VerticalAlignment" Value="Top"/>
         <Setter Property="HorizontalAlignment" Value="Right"/>
     </Style>
 
-    <Style Selector="WindowNotificationManager:bottomleft /template/ StackPanel#PART_Items">
+    <Style Selector="WindowNotificationManager:bottomleft /template/ ReversibleStackPanel#PART_Items">
         <Setter Property="ReverseOrder" Value="True"/>
         <Setter Property="VerticalAlignment" Value="Bottom"/>
         <Setter Property="HorizontalAlignment" Value="Left"/>
     </Style>
 
-    <Style Selector="WindowNotificationManager:bottomright /template/ StackPanel#PART_Items">
+    <Style Selector="WindowNotificationManager:bottomright /template/ ReversibleStackPanel#PART_Items">
         <Setter Property="ReverseOrder" Value="True"/>
         <Setter Property="VerticalAlignment" Value="Bottom"/>
         <Setter Property="HorizontalAlignment" Value="Right"/>