fix ComboBox and start working Popup

src/Avalonia.Controls/ComboBox.cs

@@ -13,6 +13,7 @@ using Avalonia.Interactivity;
 using Avalonia.Layout;
 using Avalonia.Media;
 using Avalonia.VisualTree;
+using Avalonia.LogicalTree;
 
 namespace Avalonia.Controls
 {
@@ -182,6 +183,24 @@ namespace Avalonia.Controls
             this.UpdateSelectionBoxItem(SelectedItem);
         }
 
+        // Because the SelectedItem isn't connected to the visual tree
+        public override void InvalidateFlowDirection()
+        {
+            if (SelectedItem is Control selectedControl)
+            {
+                selectedControl.InvalidateFlowDirection();
+                foreach (var logical in selectedControl.GetLogicalDescendants())
+                {
+                    if (logical is Control childControl)
+                    {
+                        childControl.InvalidateFlowDirection();
+                    }
+                }
+            }
+
+            base.InvalidateFlowDirection();
+        }
+
         /// <inheritdoc/>
         protected override void OnKeyDown(KeyEventArgs e)
         {

src/Avalonia.Controls/Control.cs

@@ -190,6 +190,7 @@ namespace Avalonia.Controls
         {
             base.OnAttachedToVisualTreeCore(e);
 
+            InvalidateFlowDirection();
             InitializeIfNeeded();
         }
 
@@ -317,25 +318,29 @@ namespace Avalonia.Controls
             if (change.Property == FlowDirectionProperty)
             {
                 // A change in value inherited should be prevented from calling this method
-                // Because it will be handled from here by NotifyDescendantFlowDirection
+                // because it will be handled from here
                 if (GetBaseValue(FlowDirectionProperty, change.Priority).HasValue)
                 {
                     InvalidateFlowDirection();
-                    NotifyDescendantFlowDirection();
+                    
+                    foreach (var visual in this.GetVisualDescendants())
+                    {
+                        if (visual is Control child)
+                        {
+                            child.InvalidateFlowDirection();
+                        }
+                    }
 
                     InvalidateVisual();
                 }
             }
         }
 
-        protected override void OnAttachedToVisualTree(VisualTreeAttachmentEventArgs e)
-        {
-            base.OnAttachedToVisualTree(e);
-
-            InvalidateFlowDirection();
-        }
-
-        private void InvalidateFlowDirection()
+        /// <summary>
+        /// Computes the <see cref="IVisual.IsMirrorTransform"/> value according to the 
+        /// <see cref="FlowDirection"/>
+        /// </summary>
+        public virtual void InvalidateFlowDirection()
         {
             bool parentShouldPresentedMirrored = false;
             bool thisShouldPresentedMirrored = ShouldPresentedMirrored();
@@ -345,23 +350,16 @@ namespace Avalonia.Controls
             {
                 parentShouldPresentedMirrored = parent.ShouldPresentedMirrored();
             }
+            else if (this.Parent is Control logicalParent)
+            {
+                parentShouldPresentedMirrored = logicalParent.ShouldPresentedMirrored();
+            }
 
             bool shouldApplyMirrorTransform = thisShouldPresentedMirrored != parentShouldPresentedMirrored;
 
             IsMirrorTransform = shouldApplyMirrorTransform;
         }
 
-        private void NotifyDescendantFlowDirection()
-        {
-            foreach (var visual in this.GetVisualDescendants())
-            {
-                if (visual is Control child)
-                {
-                    child.InvalidateFlowDirection();
-                }
-            }
-        }
-
         /// <summary>
         /// Determines whether the element should be presented mirrored, this
         /// method related to FlowDirection system and returns true if FlowDirection

src/Avalonia.Controls/MenuItem.cs

@@ -14,6 +14,8 @@ using Avalonia.Input;
 using Avalonia.Interactivity;
 using Avalonia.LogicalTree;
 using Avalonia.VisualTree;
+using Avalonia.Media;
+using Avalonia.Controls.Primitives.PopupPositioning;
 
 namespace Avalonia.Controls
 {
@@ -409,6 +411,33 @@ namespace Avalonia.Controls
             }
         }
 
+        public override void InvalidateFlowDirection()
+        {
+            if (_popup != null && Parent is Menu)
+            {
+                var popupAnchor = FlowDirection == FlowDirection.LeftToRight ? 
+                    PopupAnchor.BottomLeft : PopupAnchor.BottomRight;
+
+                var popupGravity = FlowDirection == FlowDirection.LeftToRight ? 
+                    PopupGravity.BottomRight : PopupGravity.BottomLeft;
+
+                var placement = FlowDirection == FlowDirection.LeftToRight ? 
+                    PlacementMode.Right : PlacementMode.Left;
+
+                _popup.PlacementAnchor = popupAnchor;
+                _popup.PlacementGravity = popupGravity;
+            }
+            else if (_popup != null)
+            {
+                var placement = FlowDirection == FlowDirection.LeftToRight ? 
+                    PlacementMode.Right : PlacementMode.Left;
+
+                _popup.PlacementMode = placement;
+            }
+
+            base.InvalidateFlowDirection();
+        }
+
         /// <summary>
         /// Called when the <see cref="MenuItem"/> is clicked.
         /// </summary>
@@ -493,6 +522,8 @@ namespace Avalonia.Controls
                 _popup.Opened += PopupOpened;
                 _popup.Closed += PopupClosed;
             }
+
+            InvalidateFlowDirection();
         }
 
         protected override AutomationPeer OnCreateAutomationPeer()

src/Avalonia.Controls/TopLevel.cs

@@ -530,6 +530,10 @@ namespace Avalonia.Controls
         ITextInputMethodImpl? ITextInputMethodRoot.InputMethod =>
             (PlatformImpl as ITopLevelImplWithTextInputMethod)?.TextInputMethod;
 
+        public override void InvalidateFlowDirection()
+        {
+        }
+        
         protected override bool ShouldPresentedMirrored() => false;
     }
 }

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

@@ -111,7 +111,6 @@
           </Border>
           <Popup Name="PART_Popup"
                  WindowManagerAddShadowHint="False"
-                 PlacementMode="Right"
                  HorizontalOffset="{DynamicResource MenuFlyoutSubItemPopupHorizontalOffset}"
                  IsLightDismissEnabled="False"
                  IsOpen="{TemplateBinding IsSubMenuOpen, Mode=TwoWay}">
@@ -160,6 +159,7 @@
             </ContentPresenter>
             <Popup Name="PART_Popup"
                    WindowManagerAddShadowHint="False"
+                   PlacementMode="AnchorAndGravity"
                    MinWidth="{ReflectionBinding Bounds.Width, RelativeSource={RelativeSource TemplatedParent}}"
                    IsLightDismissEnabled="True"
                    IsOpen="{TemplateBinding IsSubMenuOpen, Mode=TwoWay}"

src/Avalonia.Visuals/Visual.cs

@@ -227,7 +227,7 @@ namespace Avalonia
         }
 
         /// <summary>
-        /// Gets or sets a value indicating whether this control presented as mirror.
+        /// Gets or sets a value indicating whether to apply mirror transform on this control.
         /// </summary>
         public bool IsMirrorTransform 
         { 

src/Avalonia.Visuals/VisualTree/IVisual.cs

@@ -76,7 +76,7 @@ namespace Avalonia.VisualTree
         IBrush? OpacityMask { get; set; }
 
         /// <summary>
-        /// Gets a value indicating whether this control presented as mirror.
+        /// Gets a value indicating whether to apply mirror transform on this control.
         /// </summary>
         bool IsMirrorTransform { get; }