diff --git a/src/Avalonia.Controls.DataGrid/Collections/DataGridCollectionView.cs b/src/Avalonia.Controls.DataGrid/Collections/DataGridCollectionView.cs
index b97f2a2bcb..7c81c90d82 100644
--- a/src/Avalonia.Controls.DataGrid/Collections/DataGridCollectionView.cs
+++ b/src/Avalonia.Controls.DataGrid/Collections/DataGridCollectionView.cs
@@ -3275,7 +3275,7 @@ namespace Avalonia.Collections
                 addIndex);
 
             // next check if we need to add an item into the current group
-            // bool needsGrouping = false;
+            bool needsGrouping = false;
             if (Count == 1 && GroupDescriptions.Count > 0)
             {
                 // if this is the first item being added
@@ -3302,7 +3302,7 @@ namespace Avalonia.Collections
                 // otherwise, we need to validate that it is within the current page.
                 if (PageSize == 0 || (PageIndex + 1) * PageSize > leafIndex)
                 {
-                    //needsGrouping = true;
+                    needsGrouping = true;
 
                     int pageStartIndex = PageIndex * PageSize;
 
@@ -3340,6 +3340,13 @@ namespace Avalonia.Collections
                 }
             }
 
+            // if we need to add the item into the current group
+            // that will be displayed
+            if (needsGrouping)
+            {
+                this._group.AddToSubgroups(addedItem, false /*loading*/);
+            }
+
             int addedIndex = IndexOf(addedItem);
 
             // if the item is within the current page
diff --git a/src/Avalonia.Controls/Control.cs b/src/Avalonia.Controls/Control.cs
index f92ca83f13..4aab92c428 100644
--- a/src/Avalonia.Controls/Control.cs
+++ b/src/Avalonia.Controls/Control.cs
@@ -8,6 +8,8 @@ using Avalonia.Rendering;
 using Avalonia.Styling;
 using Avalonia.VisualTree;
 
+#nullable enable
+
 namespace Avalonia.Controls
 {
     /// <summary>
@@ -23,20 +25,20 @@ namespace Avalonia.Controls
         /// <summary>
         /// Defines the <see cref="FocusAdorner"/> property.
         /// </summary>
-        public static readonly StyledProperty<ITemplate<IControl>> FocusAdornerProperty =
-            AvaloniaProperty.Register<Control, ITemplate<IControl>>(nameof(FocusAdorner));
+        public static readonly StyledProperty<ITemplate<IControl>?> FocusAdornerProperty =
+            AvaloniaProperty.Register<Control, ITemplate<IControl>?>(nameof(FocusAdorner));
 
         /// <summary>
         /// Defines the <see cref="Tag"/> property.
         /// </summary>
-        public static readonly StyledProperty<object> TagProperty =
-            AvaloniaProperty.Register<Control, object>(nameof(Tag));
+        public static readonly StyledProperty<object?> TagProperty =
+            AvaloniaProperty.Register<Control, object?>(nameof(Tag));
         
         /// <summary>
         /// Defines the <see cref="ContextMenu"/> property.
         /// </summary>
-        public static readonly StyledProperty<ContextMenu> ContextMenuProperty =
-            AvaloniaProperty.Register<Control, ContextMenu>(nameof(ContextMenu));
+        public static readonly StyledProperty<ContextMenu?> ContextMenuProperty =
+            AvaloniaProperty.Register<Control, ContextMenu?>(nameof(ContextMenu));
 
         /// <summary>
         /// Event raised when an element wishes to be scrolled into view.
@@ -44,16 +46,16 @@ namespace Avalonia.Controls
         public static readonly RoutedEvent<RequestBringIntoViewEventArgs> RequestBringIntoViewEvent =
             RoutedEvent.Register<Control, RequestBringIntoViewEventArgs>("RequestBringIntoView", RoutingStrategies.Bubble);
 
-        private DataTemplates _dataTemplates;
-        private IControl _focusAdorner;
+        private DataTemplates? _dataTemplates;
+        private IControl? _focusAdorner;
 
         /// <summary>
         /// Gets or sets the control's focus adorner.
         /// </summary>
-        public ITemplate<IControl> FocusAdorner
+        public ITemplate<IControl>? FocusAdorner
         {
-            get { return GetValue(FocusAdornerProperty); }
-            set { SetValue(FocusAdornerProperty, value); }
+            get => GetValue(FocusAdornerProperty);
+            set => SetValue(FocusAdornerProperty, value);
         }
 
         /// <summary>
@@ -63,27 +65,27 @@ namespace Avalonia.Controls
         /// Each control may define data templates which are applied to the control itself and its
         /// children.
         /// </remarks>
-        public DataTemplates DataTemplates => _dataTemplates ?? (_dataTemplates = new DataTemplates());
+        public DataTemplates DataTemplates => _dataTemplates ??= new DataTemplates();
 
         /// <summary>
         /// Gets or sets a context menu to the control.
         /// </summary>
-        public ContextMenu ContextMenu
+        public ContextMenu? ContextMenu
         {
-            get { return GetValue(ContextMenuProperty); }
-            set { SetValue(ContextMenuProperty, value); }
+            get => GetValue(ContextMenuProperty);
+            set => SetValue(ContextMenuProperty, value);
         }
 
         /// <summary>
         /// Gets or sets a user-defined object attached to the control.
         /// </summary>
-        public object Tag
+        public object? Tag
         {
-            get { return GetValue(TagProperty); }
-            set { SetValue(TagProperty, value); }
+            get => GetValue(TagProperty);
+            set => SetValue(TagProperty, value);
         }
 
-        public new IControl Parent => (IControl)base.Parent;
+        public new IControl? Parent => (IControl?)base.Parent;
 
         /// <inheritdoc/>
         bool IDataTemplateHost.IsDataTemplatesInitialized => _dataTemplates != null;
@@ -106,15 +108,10 @@ namespace Avalonia.Controls
                     {
                         var c = i as IControl;
 
-                        if (c?.IsInitialized == false)
+                        if (c?.IsInitialized == false && c is ISupportInitialize init)
                         {
-                            var init = c as ISupportInitialize;
-
-                            if (init != null)
-                            {
-                                init.BeginInit();
-                                init.EndInit();
-                            }
+                            init.BeginInit();
+                            init.EndInit();
                         }
                     }
                 }
@@ -131,10 +128,7 @@ namespace Avalonia.Controls
         /// Gets the element that receives the focus adorner.
         /// </summary>
         /// <returns>The control that receives the focus adorner.</returns>
-        protected virtual IControl GetTemplateFocusTarget()
-        {
-            return this;
-        }
+        protected virtual IControl? GetTemplateFocusTarget() => this;
 
         /// <inheritdoc/>
         protected sealed override void OnAttachedToVisualTreeCore(VisualTreeAttachmentEventArgs e)
@@ -173,15 +167,10 @@ namespace Avalonia.Controls
                         }
                     }
 
-                    if (_focusAdorner != null)
+                    if (_focusAdorner != null && GetTemplateFocusTarget() is Visual target)
                     {
-                        var target = (Visual)GetTemplateFocusTarget();
-
-                        if (target != null)
-                        {
-                            AdornerLayer.SetAdornedElement((Visual)_focusAdorner, target);
-                            adornerLayer.Children.Add(_focusAdorner);
-                        }
+                        AdornerLayer.SetAdornedElement((Visual)_focusAdorner, target);
+                        adornerLayer.Children.Add(_focusAdorner);
                     }
                 }
             }
diff --git a/src/Avalonia.Controls/Window.cs b/src/Avalonia.Controls/Window.cs
index ab99cfad17..98f4cadc13 100644
--- a/src/Avalonia.Controls/Window.cs
+++ b/src/Avalonia.Controls/Window.cs
@@ -4,10 +4,7 @@ using System.ComponentModel;
 using System.Linq;
 using System.Reactive.Linq;
 using System.Threading.Tasks;
-using Avalonia.Controls.Chrome;
 using Avalonia.Controls.Platform;
-using Avalonia.Controls.Primitives;
-using Avalonia.Data;
 using Avalonia.Input;
 using Avalonia.Interactivity;
 using Avalonia.Layout;
@@ -482,10 +479,9 @@ namespace Avalonia.Controls
 
             try
             {
-                if (!ignoreCancel && HandleClosing())
+                if (!ignoreCancel && ShouldCancelClose())
                 {
                     close = false;
-                    return;
                 }
             }
             finally
@@ -497,11 +493,25 @@ namespace Avalonia.Controls
             }
         }
 
+        /// <summary>
+        /// Handles a closing notification from <see cref="IWindowImpl.Closing"/>.
+        /// <returns>true if closing is cancelled. Otherwise false.</returns>
+        /// </summary>
+        protected virtual bool HandleClosing()
+        {
+            if (!ShouldCancelClose())
+            {
+                CloseInternal();
+                return false;
+            }
+            
+            return true;
+        }
+
         private void CloseInternal()
         {
             foreach (var (child, _) in _children.ToList())
             {
-                // if we HandleClosing() before then there will be no children.
                 child.CloseInternal();
             }
 
@@ -515,20 +525,18 @@ namespace Avalonia.Controls
             PlatformImpl?.Dispose();
         }
 
-        /// <summary>
-        /// Handles a closing notification from <see cref="IWindowImpl.Closing"/>.
-        /// </summary>
-        protected virtual bool HandleClosing()
+        private bool ShouldCancelClose(CancelEventArgs args = null)
         {
+            if (args is null)
+            {
+                args = new CancelEventArgs();
+            }
+            
             bool canClose = true;
 
             foreach (var (child, _) in _children.ToList())
             {
-                if (!child.HandleClosing())
-                {
-                    child.CloseInternal();
-                }
-                else
+                if (child.ShouldCancelClose(args))
                 {
                     canClose = false;
                 }
@@ -536,15 +544,12 @@ namespace Avalonia.Controls
 
             if (canClose)
             {
-                var args = new CancelEventArgs();
                 OnClosing(args);
 
                 return args.Cancel;
             }
-            else
-            {
-                return !canClose;
-            }
+
+            return true;
         }
 
         protected virtual void HandleWindowStateChanged(WindowState state)
diff --git a/src/Avalonia.Layout/ElementManager.cs b/src/Avalonia.Layout/ElementManager.cs
index bf5a45966b..cb13deb15f 100644
--- a/src/Avalonia.Layout/ElementManager.cs
+++ b/src/Avalonia.Layout/ElementManager.cs
@@ -207,7 +207,7 @@ namespace Avalonia.Layout
             }
         }
 
-        public bool IsIndexValidInData(int currentIndex) => currentIndex >= 0 && currentIndex < _context.ItemCount;
+        public bool IsIndexValidInData(int currentIndex) => (uint)currentIndex < _context.ItemCount;
 
         public ILayoutable GetRealizedElement(int dataIndex)
         {
diff --git a/src/Avalonia.Visuals/Visual.cs b/src/Avalonia.Visuals/Visual.cs
index 9327531b46..80d4195421 100644
--- a/src/Avalonia.Visuals/Visual.cs
+++ b/src/Avalonia.Visuals/Visual.cs
@@ -150,7 +150,7 @@ namespace Avalonia
         public TransformedBounds? TransformedBounds => _transformedBounds;
 
         /// <summary>
-        /// Gets a value indicating whether the control should be clipped to its bounds.
+        /// Gets or sets a value indicating whether the control should be clipped to its bounds.
         /// </summary>
         public bool ClipToBounds
         {
@@ -191,7 +191,7 @@ namespace Avalonia
         }
 
         /// <summary>
-        /// Gets a value indicating whether this control is visible.
+        /// Gets or sets a value indicating whether this control is visible.
         /// </summary>
         public bool IsVisible
         {
@@ -200,7 +200,7 @@ namespace Avalonia
         }
 
         /// <summary>
-        /// Gets the opacity of the control.
+        /// Gets or sets the opacity of the control.
         /// </summary>
         public double Opacity
         {
@@ -209,7 +209,7 @@ namespace Avalonia
         }
 
         /// <summary>
-        /// Gets the opacity mask of the control.
+        /// Gets or sets the opacity mask of the control.
         /// </summary>
         public IBrush OpacityMask
         {
@@ -218,7 +218,7 @@ namespace Avalonia
         }
 
         /// <summary>
-        /// Gets the render transform of the control.
+        /// Gets or sets the render transform of the control.
         /// </summary>
         public ITransform RenderTransform
         {
@@ -227,7 +227,7 @@ namespace Avalonia
         }
 
         /// <summary>
-        /// Gets the transform origin of the control.
+        /// Gets or sets the transform origin of the control.
         /// </summary>
         public RelativePoint RenderTransformOrigin
         {
@@ -236,7 +236,7 @@ namespace Avalonia
         }
 
         /// <summary>
-        /// Gets the Z index of the control.
+        /// Gets or sets the Z index of the control.
         /// </summary>
         /// <remarks>
         /// Controls with a higher <see cref="ZIndex"/> will appear in front of controls with
diff --git a/tests/Avalonia.Controls.UnitTests/WindowTests.cs b/tests/Avalonia.Controls.UnitTests/WindowTests.cs
index ba29001cf3..e8311b79ac 100644
--- a/tests/Avalonia.Controls.UnitTests/WindowTests.cs
+++ b/tests/Avalonia.Controls.UnitTests/WindowTests.cs
@@ -1,7 +1,6 @@
 using System;
 using System.Collections.Generic;
 using System.Threading.Tasks;
-using Avalonia.Layout;
 using Avalonia.Platform;
 using Avalonia.Rendering;
 using Avalonia.UnitTests;
@@ -137,6 +136,129 @@ namespace Avalonia.Controls.UnitTests
                 Assert.Equal(1, count);
             }
         }
+        
+        [Theory]
+        [InlineData(true)]
+        [InlineData(false)]
+        public void Child_windows_should_be_closed_before_parent(bool programaticClose)
+        {
+            using (UnitTestApplication.Start(TestServices.StyledWindow))
+            {
+                var window = new Window();
+                var child = new Window();
+
+                int count = 0;
+                int windowClosing = 0;
+                int childClosing = 0;
+                int windowClosed = 0;
+                int childClosed = 0;
+
+                window.Closing += (sender, e) =>
+                {
+                    count++;
+                    windowClosing = count;
+                };
+                
+                child.Closing += (sender, e) =>
+                {
+                    count++;
+                    childClosing = count;
+                };
+                
+                window.Closed += (sender, e) =>
+                {
+                    count++;
+                    windowClosed = count;
+                };
+                
+                child.Closed += (sender, e) =>
+                {
+                    count++;
+                    childClosed = count;
+                };
+
+                window.Show();
+                child.Show(window);
+
+                if (programaticClose)
+                {
+                    window.Close();
+                }
+                else
+                {
+                    var cancel = window.PlatformImpl.Closing();
+
+                    Assert.Equal(false, cancel);
+                }
+
+                Assert.Equal(2, windowClosing);
+                Assert.Equal(1, childClosing);
+                Assert.Equal(4, windowClosed);
+                Assert.Equal(3, childClosed);
+            }
+        }
+        
+        [Theory]
+        [InlineData(true)]
+        [InlineData(false)]
+        public void Child_windows_must_not_close_before_parent_has_chance_to_Cancel_OSCloseButton(bool programaticClose)
+        {
+            using (UnitTestApplication.Start(TestServices.StyledWindow))
+            {
+                var window = new Window();
+                var child = new Window();
+
+                int count = 0;
+                int windowClosing = 0;
+                int childClosing = 0;
+                int windowClosed = 0;
+                int childClosed = 0;
+
+                window.Closing += (sender, e) =>
+                {
+                    count++;
+                    windowClosing = count;
+                    e.Cancel = true;
+                };
+                
+                child.Closing += (sender, e) =>
+                {
+                    count++;
+                    childClosing = count;
+                };
+                
+                window.Closed += (sender, e) =>
+                {
+                    count++;
+                    windowClosed = count;
+                };
+                
+                child.Closed += (sender, e) =>
+                {
+                    count++;
+                    childClosed = count;
+                };
+
+                window.Show();
+                child.Show(window);
+                
+                if (programaticClose)
+                {
+                    window.Close();
+                }
+                else
+                {
+                    var cancel = window.PlatformImpl.Closing();
+
+                    Assert.Equal(true, cancel);
+                }
+
+                Assert.Equal(2, windowClosing);
+                Assert.Equal(1, childClosing);
+                Assert.Equal(0, windowClosed);
+                Assert.Equal(0, childClosed);
+            }
+        }
 
         [Fact]
         public void Showing_Should_Start_Renderer()