Changes after review, added FlyoutPresenterTheme

samples/ControlCatalog/Pages/FlyoutsPage.axaml

@@ -16,8 +16,13 @@
             <MenuItem Header="Item 3" />
         </MenuFlyout>
         <Flyout Placement="Bottom" x:Key="BasicFlyout">
+            <Flyout.FlyoutPresenterTheme>
+                <ControlTheme TargetType="FlyoutPresenter" BasedOn="{StaticResource {x:Type FlyoutPresenter}}">
+                    <Setter Property="CornerRadius" Value="20" />
+                </ControlTheme>
+            </Flyout.FlyoutPresenterTheme>
             <Panel Width="100" Height="100">
-                <TextBlock Text="Flyout Content!" />
+                <TextBlock Text="Flyout Content with a custom presenter theme!" TextWrapping="Wrap" />
             </Panel>
         </Flyout>
     </UserControl.Resources>

src/Avalonia.Controls/Flyouts/Flyout.cs

@@ -1,5 +1,7 @@
-using Avalonia.Controls.Primitives;
+using System.ComponentModel;
+using Avalonia.Controls.Primitives;
 using Avalonia.Metadata;
+using Avalonia.Styling;
 
 namespace Avalonia.Controls
 {
@@ -18,6 +20,21 @@ namespace Avalonia.Controls
 
         private Classes? _classes;
 
+        /// <summary>
+        /// Defines the <see cref="FlyoutPresenterTheme"/> property.
+        /// </summary>
+        public static readonly StyledProperty<ControlTheme?> FlyoutPresenterThemeProperty =
+            AvaloniaProperty.Register<Flyout, ControlTheme?>(nameof(FlyoutPresenterTheme));
+        
+        /// <summary>
+        /// Gets or sets the <see cref="ControlTheme"/> that is applied to the container element generated for the flyout presenter.
+        /// </summary>
+        public ControlTheme? FlyoutPresenterTheme
+        {
+            get => GetValue(FlyoutPresenterThemeProperty); 
+            set => SetValue(FlyoutPresenterThemeProperty, value);
+        }
+        
         /// <summary>
         /// Gets or sets the content to display in this flyout
         /// </summary>
@@ -36,13 +53,22 @@ namespace Avalonia.Controls
             };
         }
 
-        protected override void OnOpened()
+        protected override void OnOpening(CancelEventArgs args)
         {
-            if (_classes != null)
+            if (Popup.Child is { } presenter)
             {
-                SetPresenterClasses(Popup.Child, FlyoutPresenterClasses);
+                if (_classes != null)
+                {
+                    SetPresenterClasses(presenter, FlyoutPresenterClasses);
+                }
+
+                if (FlyoutPresenterTheme is { } theme)
+                {
+                    presenter.SetValue(Control.ThemeProperty, theme);
+                }
             }
-            base.OnOpened();
+
+            base.OnOpening(args);
         }
     }
 }

src/Avalonia.Controls/Flyouts/MenuFlyout.cs

@@ -1,4 +1,5 @@
 using System.Collections;
+using System.ComponentModel;
 using Avalonia.Collections;
 using Avalonia.Controls.Primitives;
 using Avalonia.Controls.Templates;
@@ -34,6 +35,12 @@ namespace Avalonia.Controls
         public static readonly StyledProperty<ControlTheme?> ItemContainerThemeProperty =
             ItemsControl.ItemContainerThemeProperty.AddOwner<MenuFlyout>();
 
+        /// <summary>
+        /// Defines the <see cref="FlyoutPresenterTheme"/> property.
+        /// </summary>
+        public static readonly StyledProperty<ControlTheme?> FlyoutPresenterThemeProperty =
+            Flyout.FlyoutPresenterThemeProperty.AddOwner<MenuFlyout>();
+        
         public Classes FlyoutPresenterClasses => _classes ??= new Classes();
 
         /// <summary>
@@ -60,10 +67,19 @@ namespace Avalonia.Controls
         /// </summary>
         public ControlTheme? ItemContainerTheme
         {
-            get { return GetValue(ItemContainerThemeProperty); }
+            get => GetValue(ItemContainerThemeProperty);
-            set { SetValue(ItemContainerThemeProperty, value); }
+            set => SetValue(ItemContainerThemeProperty, value);
         }
 
+        /// <summary>
+        /// Gets or sets the <see cref="ControlTheme"/> that is applied to the container element generated for the flyout presenter.
+        /// </summary>
+        public ControlTheme? FlyoutPresenterTheme
+        {
+            get => GetValue(FlyoutPresenterThemeProperty); 
+            set => SetValue(FlyoutPresenterThemeProperty, value);
+        }
+        
         private Classes? _classes;
         private IEnumerable? _items;
         private IDataTemplate? _itemTemplate;
@@ -78,13 +94,22 @@ namespace Avalonia.Controls
             };
         }
 
-        protected override void OnOpened()
+        protected override void OnOpening(CancelEventArgs args)
         {
-            if (_classes != null)
+            if (Popup.Child is { } presenter)
             {
-                SetPresenterClasses(Popup.Child, FlyoutPresenterClasses);
+                if (_classes != null)
+                {
+                    SetPresenterClasses(presenter, FlyoutPresenterClasses);
+                }
+
+                if (FlyoutPresenterTheme is { } theme)
+                {
+                    presenter.SetValue(Control.ThemeProperty, theme);
+                }
             }
-            base.OnOpened();
+
+            base.OnOpening(args);
         }
     }
 }

src/Avalonia.Controls/Flyouts/PopupFlyoutBase.cs

@@ -37,9 +37,8 @@ namespace Avalonia.Controls.Primitives
         /// <summary>
         /// Defines the <see cref="ShowMode"/> property
         /// </summary>
-        public static readonly DirectProperty<PopupFlyoutBase, FlyoutShowMode> ShowModeProperty =
+        public static readonly StyledProperty<FlyoutShowMode> ShowModeProperty =
-            AvaloniaProperty.RegisterDirect<PopupFlyoutBase, FlyoutShowMode>(nameof(ShowMode),
+            AvaloniaProperty.Register<PopupFlyoutBase, FlyoutShowMode>(nameof(ShowMode));
-                x => x.ShowMode, (x, v) => x.ShowMode = v);
 
         /// <summary>
         /// Defines the <see cref="OverlayInputPassThroughElement"/> property
@@ -50,7 +49,6 @@ namespace Avalonia.Controls.Primitives
                 (o, v) => o._overlayInputPassThroughElement = v);
         
         private readonly Lazy<Popup> _popupLazy;
-        private FlyoutShowMode _showMode = FlyoutShowMode.Standard;
         private Rect? _enlargedPopupRect;
         private PixelRect? _enlargePopupRectScreenPixelRect;
         private IDisposable? _transientDisposable;
@@ -81,39 +79,38 @@ namespace Avalonia.Controls.Primitives
         /// <inheritdoc cref="Popup.PlacementGravity"/>
         public PopupGravity PlacementGravity
         {
-            get { return GetValue(PlacementGravityProperty); }
+            get => GetValue(PlacementGravityProperty);
-            set { SetValue(PlacementGravityProperty, value); }
+            set => SetValue(PlacementGravityProperty, value);
         }
 
         /// <inheritdoc cref="Popup.PlacementAnchor"/>
         public PopupAnchor PlacementAnchor
         {
-            get { return GetValue(PlacementAnchorProperty); }
+            get => GetValue(PlacementAnchorProperty);
-            set { SetValue(PlacementAnchorProperty, value); }
+            set => SetValue(PlacementAnchorProperty, value);
         }
 
         /// <inheritdoc cref="Popup.HorizontalOffset"/>
         public double HorizontalOffset
         {
-            get { return GetValue(HorizontalOffsetProperty); }
+            get => GetValue(HorizontalOffsetProperty);
-            set { SetValue(HorizontalOffsetProperty, value); }
+            set => SetValue(HorizontalOffsetProperty, value);
         }
 
         /// <inheritdoc cref="Popup.VerticalOffset"/>
         public double VerticalOffset
         {
-            get { return GetValue(VerticalOffsetProperty); }
+            get => GetValue(VerticalOffsetProperty);
-            set { SetValue(VerticalOffsetProperty, value); }
+            set => SetValue(VerticalOffsetProperty, value);
         }
 
-        
         /// <summary>
         /// Gets or sets the desired ShowMode
         /// </summary>
         public FlyoutShowMode ShowMode
         {
-            get => _showMode;
+            get => GetValue(ShowModeProperty);
-            set => SetAndRaise(ShowModeProperty, ref _showMode, value);
+            set => SetValue(ShowModeProperty, value);
         }
 
         /// <summary>