Don't leak subs to CanExecuteChanged in MenuItem.

src/Avalonia.Controls/MenuItem.cs

@@ -286,6 +286,26 @@ namespace Avalonia.Controls
             return new MenuItemContainerGenerator(this);
         }
 
+        protected override void OnAttachedToLogicalTree(LogicalTreeAttachmentEventArgs e)
+        {
+            base.OnAttachedToLogicalTree(e);
+
+            if (Command != null)
+            {
+                Command.CanExecuteChanged += CanExecuteChanged;
+            }
+        }
+
+        protected override void OnDetachedFromLogicalTree(LogicalTreeAttachmentEventArgs e)
+        {
+            base.OnAttachedToLogicalTree(e);
+
+            if (Command != null)
+            {
+                Command.CanExecuteChanged -= CanExecuteChanged;
+            }
+        }
+
         /// <summary>
         /// Called when the <see cref="MenuItem"/> is clicked.
         /// </summary>
@@ -399,14 +419,17 @@ namespace Avalonia.Controls
         {
             if (e.Sender is MenuItem menuItem)
             {
-                if (e.OldValue is ICommand oldCommand)
+                if (((ILogical)menuItem).IsAttachedToLogicalTree)
                 {
-                    oldCommand.CanExecuteChanged -= menuItem.CanExecuteChanged;
-                }
+                    if (e.OldValue is ICommand oldCommand)
+                    {
+                        oldCommand.CanExecuteChanged -= menuItem.CanExecuteChanged;
+                    }
 
-                if (e.NewValue is ICommand newCommand)
-                {
-                    newCommand.CanExecuteChanged += menuItem.CanExecuteChanged;
+                    if (e.NewValue is ICommand newCommand)
+                    {
+                        newCommand.CanExecuteChanged += menuItem.CanExecuteChanged;
+                    }
                 }
 
                 menuItem.CanExecuteChanged(menuItem, EventArgs.Empty);

tests/Avalonia.Controls.UnitTests/MenuItemTests.cs

@@ -1,6 +1,8 @@
 using System;
 using System.Collections.Generic;
 using System.Text;
+using System.Windows.Input;
+using Avalonia.UnitTests;
 using Xunit;
 
 namespace Avalonia.Controls.UnitTests
@@ -22,5 +24,57 @@ namespace Avalonia.Controls.UnitTests
 
             Assert.False(target.Focusable);
         }
+
+        [Fact]
+        public void MenuItem_Does_Not_Subscribe_To_Command_CanExecuteChanged_Until_Added_To_Logical_Tree()
+        {
+            var command = new TestCommand();
+            var target = new MenuItem
+            {
+                Command = command,
+            };
+
+            Assert.Equal(0, command.SubscriptionCount);
+        }
+
+        [Fact]
+        public void MenuItem_Subscribes_To_Command_CanExecuteChanged_When_Added_To_Logical_Tree()
+        {
+            var command = new TestCommand();
+            var target = new MenuItem { Command = command };
+            var root = new TestRoot { Child = target };
+
+            Assert.Equal(1, command.SubscriptionCount);
+        }
+
+        [Fact]
+        public void MenuItem_Unsubscribes_From_Command_CanExecuteChanged_When_Removed_From_Logical_Tree()
+        {
+            var command = new TestCommand();
+            var target = new MenuItem { Command = command };
+            var root = new TestRoot { Child = target };
+
+            root.Child = null;
+            Assert.Equal(0, command.SubscriptionCount);
+        }
+
+        private class TestCommand : ICommand
+        {
+            private EventHandler _canExecuteChanged;
+
+            public int SubscriptionCount { get; private set; }
+
+            public event EventHandler CanExecuteChanged
+            {
+                add { _canExecuteChanged += value; ++SubscriptionCount; }
+                remove { _canExecuteChanged -= value; --SubscriptionCount; }
+            }
+
+            public bool CanExecute(object parameter) => true;
+
+            public void Execute(object parameter)
+            {
+            }
+        }
     }
 }