Merge branch 'master' into refactor/remove_duplicate_RoundLayoutValue

samples/ControlCatalog/Pages/ItemsRepeaterPage.xaml

@@ -16,6 +16,8 @@
       <Button Command="{Binding AddItem}">Add Item</Button>
       <Button Command="{Binding RandomizeHeights}">Randomize Heights</Button>
       <Button Command="{Binding ResetItems}">Reset items</Button>
+      <Button x:Name="scrollToLast">Scroll to Last</Button>
+      <Button x:Name="scrollToRandom">Scroll to Random</Button>
     </StackPanel>
     <Border BorderThickness="1" BorderBrush="{DynamicResource ThemeBorderMidBrush}" Margin="0 0 0 16">
       <ScrollViewer Name="scroller"
@@ -24,7 +26,10 @@
         <ItemsRepeater Name="repeater" Background="Transparent" Items="{Binding Items}">
           <ItemsRepeater.ItemTemplate>
             <DataTemplate>
-              <TextBlock Focusable="True" Height="{Binding Height}" Text="{Binding Text}"/>
+              <TextBlock Focusable="True"
+                         Background="{Binding Background}"
+                         Height="{Binding Height}"
+                         Text="{Binding Text}"/>
             </DataTemplate>
           </ItemsRepeater.ItemTemplate>
         </ItemsRepeater>

samples/ControlCatalog/Pages/ItemsRepeaterPage.xaml.cs

@@ -5,23 +5,32 @@ using Avalonia.Controls.Primitives;
 using Avalonia.Input;
 using Avalonia.Layout;
 using Avalonia.Markup.Xaml;
+using Avalonia.VisualTree;
 using ControlCatalog.ViewModels;
 
 namespace ControlCatalog.Pages
 {
     public class ItemsRepeaterPage : UserControl
     {
+        private readonly ItemsRepeaterPageViewModel _viewModel;
         private ItemsRepeater _repeater;
         private ScrollViewer _scroller;
+        private Button _scrollToLast;
+        private Button _scrollToRandom;
+        private Random _random = new Random(0);
 
         public ItemsRepeaterPage()
         {
             this.InitializeComponent();
             _repeater = this.FindControl<ItemsRepeater>("repeater");
             _scroller = this.FindControl<ScrollViewer>("scroller");
+            _scrollToLast = this.FindControl<Button>("scrollToLast");
+            _scrollToRandom = this.FindControl<Button>("scrollToRandom");
             _repeater.PointerPressed += RepeaterClick;
             _repeater.KeyDown += RepeaterOnKeyDown;
-            DataContext = new ItemsRepeaterPageViewModel();
+            _scrollToLast.Click += scrollToLast_Click;
+            _scrollToRandom.Click += scrollToRandom_Click;
+            DataContext = _viewModel = new ItemsRepeaterPageViewModel();
         }
 
         private void InitializeComponent()
@@ -73,18 +82,37 @@ namespace ControlCatalog.Pages
             }
         }
 
+        private void ScrollTo(int index)
+        {
+            System.Diagnostics.Debug.WriteLine("Scroll to " + index);
+            var layoutManager = ((Window)this.GetVisualRoot()).LayoutManager;
+            var element = _repeater.GetOrCreateElement(index);
+            layoutManager.ExecuteLayoutPass();
+            element.BringIntoView();
+        }
+
         private void RepeaterClick(object sender, PointerPressedEventArgs e)
         {
             var item = (e.Source as TextBlock)?.DataContext as ItemsRepeaterPageViewModel.Item;
-            ((ItemsRepeaterPageViewModel)DataContext).SelectedItem = item;
+            _viewModel.SelectedItem = item;
         }
 
         private void RepeaterOnKeyDown(object sender, KeyEventArgs e)
         {
             if (e.Key == Key.F5)
             {
-                ((ItemsRepeaterPageViewModel)DataContext).ResetItems();
+                _viewModel.ResetItems();
             }
         }
+
+        private void scrollToLast_Click(object sender, Avalonia.Interactivity.RoutedEventArgs e)
+        {
+            ScrollTo(_viewModel.Items.Count - 1);
+        }
+
+        private void scrollToRandom_Click(object sender, Avalonia.Interactivity.RoutedEventArgs e)
+        {
+            ScrollTo(_random.Next(_viewModel.Items.Count - 1));
+        }
     }
 }

samples/ControlCatalog/ViewModels/ItemsRepeaterPageViewModel.cs

@@ -1,6 +1,7 @@
 using System;
 using System.Collections.ObjectModel;
 using System.Linq;
+using Avalonia.Media;
 using ReactiveUI;
 
 namespace ControlCatalog.ViewModels
@@ -27,7 +28,7 @@ namespace ControlCatalog.ViewModels
         public void AddItem()
         {
             var index = SelectedItem != null ? Items.IndexOf(SelectedItem) : -1;
-            Items.Insert(index + 1, new Item { Text = $"New Item {_newItemIndex++}" });
+            Items.Insert(index + 1, new Item(index + 1) { Text = $"New Item {_newItemIndex++}" });
         }
 
         public void RandomizeHeights()
@@ -52,7 +53,7 @@ namespace ControlCatalog.ViewModels
             _newGenerationIndex++;
 
             return new ObservableCollection<Item>(
-                Enumerable.Range(1, 100000).Select(i => new Item
+                Enumerable.Range(1, 100000).Select(i => new Item(i)
                 {
                     Text = $"Item {i.ToString()} {suffix}"
                 }));
@@ -61,6 +62,12 @@ namespace ControlCatalog.ViewModels
         public class Item : ReactiveObject
         {
             private double _height = double.NaN;
+            private int _index;
+
+            public Item(int index)
+            {
+                _index = index;
+            }
 
             public string Text { get; set; }
             
@@ -69,6 +76,8 @@ namespace ControlCatalog.ViewModels
                 get => _height;
                 set => this.RaiseAndSetIfChanged(ref _height, value);
             }
+
+            public IBrush Background => ((_index % 2) == 0) ? Brushes.Yellow : Brushes.Wheat;
         }
     }
 }

src/Avalonia.Controls/IScrollAnchorProvider.cs

@@ -1,9 +1,29 @@
 namespace Avalonia.Controls
 {
+    /// <summary>
+    /// Specifies a contract for a scrolling control that supports scroll anchoring.
+    /// </summary>
     public interface IScrollAnchorProvider
     {
+        /// <summary>
+        /// The currently chosen anchor element to use for scroll anchoring.
+        /// </summary>
         IControl CurrentAnchor { get; }
+
+        /// <summary>
+        /// Registers a control as a potential scroll anchor candidate.
+        /// </summary>
+        /// <param name="element">
+        /// A control within the subtree of the <see cref="IScrollAnchorProvider"/>.
+        /// </param>
         void RegisterAnchorCandidate(IControl element);
+
+        /// <summary>
+        /// Unregisters a control as a potential scroll anchor candidate.
+        /// </summary>
+        /// <param name="element">
+        /// A control within the subtree of the <see cref="IScrollAnchorProvider"/>.
+        /// </param>
         void UnregisterAnchorCandidate(IControl element);
     }
 }

src/Avalonia.Controls/Presenters/ScrollContentPresenter.cs

@@ -3,10 +3,8 @@ using System.Collections.Generic;
 using System.Linq;
 using System.Reactive.Disposables;
 using System.Reactive.Linq;
-using System.Runtime.InteropServices.ComTypes;
 using Avalonia.Controls.Primitives;
 using Avalonia.Input;
-using Avalonia.LogicalTree;
 using Avalonia.VisualTree;
 
 namespace Avalonia.Controls.Presenters
@@ -14,7 +12,7 @@ namespace Avalonia.Controls.Presenters
     /// <summary>
     /// Presents a scrolling view of content inside a <see cref="ScrollViewer"/>.
     /// </summary>
-    public class ScrollContentPresenter : ContentPresenter, IPresenter, IScrollable
+    public class ScrollContentPresenter : ContentPresenter, IPresenter, IScrollable, IScrollAnchorProvider
     {
         /// <summary>
         /// Defines the <see cref="CanHorizontallyScroll"/> property.
@@ -58,13 +56,19 @@ namespace Avalonia.Controls.Presenters
                 o => o.Viewport,
                 (o, v) => o.Viewport = v);
 
+        // Arbitrary chosen value, probably need to ask ILogicalScrollable
+        private const int LogicalScrollItemSize = 50;
+
         private bool _canHorizontallyScroll;
         private bool _canVerticallyScroll;
+        private bool _arranging;
         private Size _extent;
         private Vector _offset;
         private IDisposable _logicalScrollSubscription;
         private Size _viewport;
         private Dictionary<int, Vector> _activeLogicalGestureScrolls;
+        private List<IControl> _anchorCandidates;
+        private (IControl control, Rect bounds) _anchor;
 
         /// <summary>
         /// Initializes static members of the <see cref="ScrollContentPresenter"/> class.
@@ -73,7 +77,6 @@ namespace Avalonia.Controls.Presenters
         {
             ClipToBoundsProperty.OverrideDefaultValue(typeof(ScrollContentPresenter), true);
             ChildProperty.Changed.AddClassHandler<ScrollContentPresenter>((x, e) => x.ChildChanged(e));
-            AffectsArrange<ScrollContentPresenter>(OffsetProperty);
         }
 
         /// <summary>
@@ -87,6 +90,8 @@ namespace Avalonia.Controls.Presenters
             this.GetObservable(ChildProperty).Subscribe(UpdateScrollableSubscription);
         }
 
+        internal event EventHandler<VectorEventArgs> PreArrange;
+
         /// <summary>
         /// Gets or sets a value indicating whether the content can be scrolled horizontally.
         /// </summary>
@@ -120,7 +125,7 @@ namespace Avalonia.Controls.Presenters
         public Vector Offset
         {
             get { return _offset; }
-            set { SetAndRaise(OffsetProperty, ref _offset, value); }
+            set { SetAndRaise(OffsetProperty, ref _offset, ScrollViewer.CoerceOffset(Extent, Viewport, value)); }
         }
 
         /// <summary>
@@ -132,6 +137,9 @@ namespace Avalonia.Controls.Presenters
             private set { SetAndRaise(ViewportProperty, ref _viewport, value); }
         }
 
+        /// <inheritdoc/>
+        IControl IScrollAnchorProvider.CurrentAnchor => _anchor.control;
+
         /// <summary>
         /// Attempts to bring a portion of the target visual into view by scrolling the content.
         /// </summary>
@@ -196,6 +204,30 @@ namespace Avalonia.Controls.Presenters
             return result;
         }
 
+        /// <inheritdoc/>
+        void IScrollAnchorProvider.RegisterAnchorCandidate(IControl element)
+        {
+            if (!this.IsVisualAncestorOf(element))
+            {
+                throw new InvalidOperationException(
+                    "An anchor control must be a visual descendent of the ScrollContentPresenter.");
+            }
+
+            _anchorCandidates ??= new List<IControl>();
+            _anchorCandidates.Add(element);
+        }
+
+        /// <inheritdoc/>
+        void IScrollAnchorProvider.UnregisterAnchorCandidate(IControl element)
+        {
+            _anchorCandidates?.Remove(element);
+
+            if (_anchor.control == element)
+            {
+                _anchor = default;
+            }
+        }
+
         /// <inheritdoc/>
         protected override Size MeasureOverride(Size availableSize)
         {
@@ -215,22 +247,85 @@ namespace Avalonia.Controls.Presenters
         /// <inheritdoc/>
         protected override Size ArrangeOverride(Size finalSize)
         {
+            PreArrange?.Invoke(this, new VectorEventArgs
+            {
+                Vector = new Vector(finalSize.Width, finalSize.Height),
+            });
+
             if (_logicalScrollSubscription != null || Child == null)
             {
                 return base.ArrangeOverride(finalSize);
             }
 
+            try
+            {
+                _arranging = true;
+                return ArrangeWithAnchoring(finalSize);
+            }
+            finally
+            {
+                _arranging = false;
+            }
+        }
+
+        private Size ArrangeWithAnchoring(Size finalSize)
+        {
             var size = new Size(
                 CanHorizontallyScroll ? Math.Max(Child.DesiredSize.Width, finalSize.Width) : finalSize.Width,
                 CanVerticallyScroll ? Math.Max(Child.DesiredSize.Height, finalSize.Height) : finalSize.Height);
+
+            Vector TrackAnchor()
+            {
+                // If we have an anchor and its position relative to Child has changed during the
+                // arrange then that change wasn't just due to scrolling (as scrolling doesn't adjust
+                // relative positions within Child).
+                if (_anchor.control != null &&
+                    TranslateBounds(_anchor.control, Child, out var updatedBounds) &&
+                    updatedBounds.Position != _anchor.bounds.Position)
+                {
+                    var offset = updatedBounds.Position - _anchor.bounds.Position;
+                    return offset;
+                }
+
+                return default;
+            }
+
+            // Calculate the new anchor element.
+            _anchor = CalculateCurrentAnchor();
+
+            // Do the arrange.
             ArrangeOverrideImpl(size, -Offset);
+
+            // If the anchor moved during the arrange, we need to adjust the offset and do another arrange.
+            var anchorShift = TrackAnchor();
+
+            if (anchorShift != default)
+            {
+                var newOffset = Offset + anchorShift;
+                var newExtent = Extent;
+                var maxOffset = new Vector(Extent.Width - Viewport.Width, Extent.Height - Viewport.Height);
+
+                if (newOffset.X > maxOffset.X)
+                {
+                    newExtent = newExtent.WithWidth(newOffset.X + Viewport.Width);
+                }
+
+                if (newOffset.Y > maxOffset.Y)
+                {
+                    newExtent = newExtent.WithHeight(newOffset.Y + Viewport.Height);
+                }
+
+                Extent = newExtent;
+                Offset = newOffset;
+                ArrangeOverrideImpl(size, -Offset);
+            }
+
             Viewport = finalSize;
             Extent = Child.Bounds.Size.Inflate(Child.Margin);
+
             return finalSize;
         }
 
-        // Arbitrary chosen value, probably need to ask ILogicalScrollable
-        private const int LogicalScrollItemSize = 50;
         private void OnScrollGesture(object sender, ScrollGestureEventArgs e)
         {
             if (Extent.Height > Viewport.Height || Extent.Width > Viewport.Width)
@@ -327,6 +422,16 @@ namespace Avalonia.Controls.Presenters
             }
         }
 
+        protected override void OnPropertyChanged<T>(AvaloniaPropertyChangedEventArgs<T> change)
+        {
+            if (change.Property == OffsetProperty && !_arranging)
+            {
+                InvalidateArrange();
+            }
+
+            base.OnPropertyChanged(change);
+        }
+
         private void BringIntoViewRequested(object sender, RequestBringIntoViewEventArgs e)
         {
             e.Handled = BringDescendantIntoView(e.TargetObject, e.TargetRect);
@@ -390,5 +495,84 @@ namespace Avalonia.Controls.Presenters
                 Offset = scrollable.Offset;
             }
         }
+
+        private (IControl, Rect) CalculateCurrentAnchor()
+        {
+            if (_anchorCandidates == null)
+            {
+                return default;
+            }
+
+            var bestCandidate = default(IControl);
+            var bestCandidateDistance = double.MaxValue;
+
+            // Find the anchor candidate that is scrolled closest to the top-left of this
+            // ScrollContentPresenter.
+            foreach (var element in _anchorCandidates)
+            {
+                if (element.IsVisible && GetViewportBounds(element, out var bounds))
+                {
+                    var distance = (Vector)bounds.Position;
+                    var candidateDistance = Math.Abs(distance.Length);
+
+                    if (candidateDistance < bestCandidateDistance)
+                    {
+                        bestCandidate = element;
+                        bestCandidateDistance = candidateDistance;
+                    }
+                }
+            }
+
+            if (bestCandidate != null)
+            {
+                // We have a candidate, calculate its bounds relative to Child. Because these
+                // bounds aren't relative to the ScrollContentPresenter itself, if they change
+                // then we know it wasn't just due to scrolling.
+                var unscrolledBounds = TranslateBounds(bestCandidate, Child);
+                return (bestCandidate, unscrolledBounds);
+            }
+
+            return default;
+        }
+
+        private bool GetViewportBounds(IControl element, out Rect bounds)
+        {
+            if (TranslateBounds(element, Child, out var childBounds))
+            {
+                // We want the bounds relative to the new Offset, regardless of whether the child
+                // control has actually been arranged to this offset yet, so translate first to the
+                // child control and then apply Offset rather than translating directly to this
+                // control.
+                var thisBounds = new Rect(Bounds.Size);
+                bounds = new Rect(childBounds.Position - Offset, childBounds.Size);
+                return bounds.Intersects(thisBounds);
+            }
+
+            bounds = default;
+            return false;
+        }
+
+        private Rect TranslateBounds(IControl control, IControl to)
+        {
+            if (TranslateBounds(control, to, out var bounds))
+            {
+                return bounds;
+            }
+
+            throw new InvalidOperationException("The control's bounds could not be translated to the requested control.");
+        }
+
+        private bool TranslateBounds(IControl control, IControl to, out Rect bounds)
+        {
+            if (!control.IsVisible)
+            {
+                bounds = default;
+                return false;
+            }
+
+            var p = control.TranslatePoint(default, to);
+            bounds = p.HasValue ? new Rect(p.Value, control.Bounds.Size) : default;
+            return p.HasValue;
+        }
     }
 }

src/Avalonia.Controls/Repeater/ItemsRepeater.cs

@@ -10,6 +10,7 @@ using Avalonia.Controls.Templates;
 using Avalonia.Data;
 using Avalonia.Input;
 using Avalonia.Layout;
+using Avalonia.Logging;
 using Avalonia.VisualTree;
 
 namespace Avalonia.Controls
@@ -80,6 +81,7 @@ namespace Avalonia.Controls
         static ItemsRepeater()
         {
             ClipToBoundsProperty.OverrideDefaultValue<ItemsRepeater>(true);
+            RequestBringIntoViewEvent.AddClassHandler<ItemsRepeater>((x, e) => x.OnRequestBringIntoView(e));
         }
 
         /// <summary>
@@ -305,6 +307,7 @@ namespace Avalonia.Controls
                             virtInfo.AutoRecycleCandidate &&
                             !virtInfo.KeepAlive)
                         {
+                            Logger.TryGet(LogEventLevel.Verbose, "Repeater")?.Log(this, "AutoClear - {Index}", virtInfo.Index);
                             ClearElementImpl(element);
                         }
                     }
@@ -743,6 +746,11 @@ namespace Avalonia.Controls
             }
         }
 
+        private void OnRequestBringIntoView(RequestBringIntoViewEventArgs e)
+        {
+            _viewportManager.OnBringIntoViewRequested(e);
+        }
+
         private void InvalidateMeasureForLayout(object sender, EventArgs e) => InvalidateMeasure();
 
         private void InvalidateArrangeForLayout(object sender, EventArgs e) => InvalidateArrange();

src/Avalonia.Controls/Repeater/RepeaterLayoutContext.cs

@@ -7,6 +7,7 @@ using System;
 using System.Collections.Generic;
 using System.Text;
 using Avalonia.Layout;
+using Avalonia.Logging;
 
 namespace Avalonia.Controls
 {
@@ -58,7 +59,11 @@ namespace Avalonia.Controls
 
         protected override object GetItemAtCore(int index) => _owner.ItemsSourceView.GetAt(index);
 
-        protected override void RecycleElementCore(ILayoutable element) => _owner.ClearElementImpl((IControl)element);
+        protected override void RecycleElementCore(ILayoutable element)
+        {
+            Logger.TryGet(LogEventLevel.Verbose, "Repeater")?.Log(this, "RepeaterLayout - RecycleElement: {Index}", _owner.GetElementIndex((IControl)element));
+            _owner.ClearElementImpl((IControl)element);
+        }
 
         protected override Rect RealizationRectCore() => _owner.RealizationWindow;
     }

src/Avalonia.Controls/Repeater/ViewManager.cs

@@ -11,6 +11,7 @@ using Avalonia.Controls.Templates;
 using Avalonia.Input;
 using Avalonia.Interactivity;
 using Avalonia.Layout;
+using Avalonia.Logging;
 using Avalonia.VisualTree;
 
 namespace Avalonia.Controls
@@ -60,11 +61,13 @@ namespace Avalonia.Controls
             if (suppressAutoRecycle)
             {
                 virtInfo.AutoRecycleCandidate = false;
+                Logger.TryGet(LogEventLevel.Verbose, "Repeater")?.Log(this, "GetElement: {Index} Not AutoRecycleCandidate:", virtInfo.Index);
             }
             else
             {
                 virtInfo.AutoRecycleCandidate = true;
                 virtInfo.KeepAlive = true;
+                Logger.TryGet(LogEventLevel.Verbose, "Repeater")?.Log(this, "GetElement: {Index} AutoRecycleCandidate:", virtInfo.Index);
             }
 
             return element;
@@ -107,10 +110,28 @@ namespace Avalonia.Controls
             }
         }
 
+        // We need to clear the datacontext to prevent crashes from happening,
+        //  however we only do that if we were the ones setting it.
+        // That is when one of the following is the case (numbering taken from line ~642):
+        // 1.2    No ItemTemplate, data is not a UIElement
+        // 2.1    ItemTemplate, data is not FrameworkElement
+        // 2.2.2  Itemtemplate, data is FrameworkElement, ElementFactory returned Element different to data
+        //
+        // In all of those three cases, we the ItemTemplateShim is NOT null.
+        // Luckily when we create the items, we store whether we were the once setting the DataContext.
         public void ClearElementToElementFactory(IControl element)
         {
             _owner.OnElementClearing(element);
 
+            var virtInfo = ItemsRepeater.GetVirtualizationInfo(element);
+            virtInfo.MoveOwnershipToElementFactory();
+
+            // During creation of this object, we were the one setting the DataContext, so clear it now.
+            if (virtInfo.MustClearDataContext)
+            {
+                element.DataContext = null;
+            }
+
             if (_owner.ItemTemplateShim != null)
             {
                 _owner.ItemTemplateShim.RecycleElement(_owner, element);
@@ -124,9 +145,6 @@ namespace Avalonia.Controls
                 }
             }
 
-            var virtInfo = ItemsRepeater.GetVirtualizationInfo(element);
-            virtInfo.MoveOwnershipToElementFactory();
-
             if (_lastFocusedElement == element)
             {
                 // Focused element is going away. Remove the tracked last focused element
@@ -594,11 +612,14 @@ namespace Avalonia.Controls
             {
                 virtInfo = ItemsRepeater.CreateAndInitializeVirtualizationInfo(element);
             }
+            // Clear flag
+            virtInfo.MustClearDataContext = false;
 
             if (data != element)
             {
                 // Prepare the element
                 element.DataContext = data;
+                virtInfo.MustClearDataContext = true;
             }
 
             virtInfo.MoveOwnershipToLayoutFromElementFactory(

src/Avalonia.Controls/Repeater/ViewportManager.cs

@@ -5,9 +5,15 @@
 
 using System;
 using System.Collections.Generic;
+using System.Reactive.Disposables;
 using System.Reactive.Linq;
 using System.Threading.Tasks;
+using Avalonia.Controls.Presenters;
+using Avalonia.Input;
 using Avalonia.Layout;
+using Avalonia.Logging;
+using Avalonia.Media;
+using Avalonia.Reactive;
 using Avalonia.Threading;
 using Avalonia.VisualTree;
 
@@ -35,8 +41,8 @@ namespace Avalonia.Controls
         // actually happened. This can happen in cases where no scrollviewer
         // in the parent chain can scroll in the shift direction.
         private Point _unshiftableShift;
-        private double _maximumHorizontalCacheLength = 0.0;
-        private double _maximumVerticalCacheLength = 0.0;
+        private double _maximumHorizontalCacheLength = 2.0;
+        private double _maximumVerticalCacheLength = 2.0;
         private double _horizontalCacheBufferPerSide;
         private double _verticalCacheBufferPerSide;
         private bool _isBringIntoViewInProgress;
@@ -184,6 +190,9 @@ namespace Avalonia.Controls
             // We tolerate viewport imprecisions up to 1 pixel to avoid invaliding layout too much.
             if (Math.Abs(_expectedViewportShift.X) > 1 || Math.Abs(_expectedViewportShift.Y) > 1)
             {
+                Logger.TryGet(LogEventLevel.Verbose, "Repeater")?.Log(this, "{LayoutId}: Expecting viewport shift of ({Shift})",
+                    _owner.Layout.LayoutId, _expectedViewportShift);
+
                 // There are cases where we might be expecting a shift but not get it. We will
                 // be waiting for the effective viewport event but if the scroll viewer is not able
                 // to perform the shift (perhaps because it cannot scroll in negative offset),
@@ -229,14 +238,12 @@ namespace Avalonia.Controls
 
         public void OnElementPrepared(IControl element)
         {
-            // If we have an anchor element, we do not want the
-            // scroll anchor provider to start anchoring some other element.
-            ////element.CanBeScrollAnchor(true);
+            _scroller?.RegisterAnchorCandidate(element);
         }
 
-        public void OnElementCleared(ILayoutable element)
+        public void OnElementCleared(IControl element)
         {
-            ////element.CanBeScrollAnchor(false);
+            _scroller?.UnregisterAnchorCandidate(element);
         }
 
         public void OnOwnerMeasuring()
@@ -282,6 +289,7 @@ namespace Avalonia.Controls
         private void OnLayoutUpdated(object sender, EventArgs args)
         {
             _owner.LayoutUpdated -= OnLayoutUpdated;
+            _layoutUpdatedSubscribed = false;
             if (_managingViewportDisabled)
             {
                 return;
@@ -293,6 +301,10 @@ namespace Avalonia.Controls
             // that can scroll in the direction where the shift is expected.
             if (_pendingViewportShift.X != 0 || _pendingViewportShift.Y != 0)
             {
+                Logger.TryGet(LogEventLevel.Verbose, "Repeater")?.Log(this, "{LayoutId}: Layout Updated with pending shift {Shift}- invalidating measure",
+                    _owner.Layout.LayoutId,
+                    _pendingViewportShift);
+
                 // Assume this is never going to come.
                 _unshiftableShift = new Point(
                     _unshiftableShift.X + _pendingViewportShift.X,
@@ -306,8 +318,11 @@ namespace Avalonia.Controls
 
         public void OnMakeAnchor(IControl anchor, bool isAnchorOutsideRealizedRange)
         {
-            _makeAnchorElement = anchor;
-            _isAnchorOutsideRealizedRange = isAnchorOutsideRealizedRange;
+            if (_makeAnchorElement != anchor)
+            {
+                _makeAnchorElement = anchor;
+                _isAnchorOutsideRealizedRange = isAnchorOutsideRealizedRange;
+            }
         }
 
         public void OnBringIntoViewRequested(RequestBringIntoViewEventArgs args)
@@ -328,26 +343,28 @@ namespace Avalonia.Controls
                 // Make sure that only the target child can be the anchor during the bring into view operation.
                 foreach (var child in _owner.Children)
                 {
-                    ////if (child.CanBeScrollAnchor && child != targetChild)
-                    ////{
-                    ////    child.CanBeScrollAnchor = false;
-                    ////}
+                    if (child != targetChild)
+                    {
+                        _scroller.UnregisterAnchorCandidate(child);
+                    }
                 }
 
-                // Register to rendering event to go back to how things were before where any child can be the anchor.
-                _isBringIntoViewInProgress = true;
-                ////if (!m_renderingToken)
-                ////{
-                ////    winrt::Windows::UI::Xaml::Media::CompositionTarget compositionTarget{ nullptr };
-                ////    m_renderingToken = compositionTarget.Rendering(winrt::auto_revoke, { this, &ViewportManagerWithPlatformFeatures::OnCompositionTargetRendering });
-                ////}
+                // Register action to go back to how things were before where any child can be the anchor. Here,
+                // WinUI uses CompositionTarget.Rendering but we don't currently have that, so post an action to
+                // run *after* rendering has completed (priority needs to be lower than Render as Transformed
+                // bounds must have been set in order for OnEffectiveViewportChanged to trigger).
+                if (!_isBringIntoViewInProgress)
+                {
+                    _isBringIntoViewInProgress = true;
+                    Dispatcher.UIThread.Post(OnCompositionTargetRendering, DispatcherPriority.Loaded);
+                }
             }
         }
 
         private IControl GetImmediateChildOfRepeater(IControl descendant)
         {
             var targetChild = descendant;
-            var parent = descendant.Parent;
+            var parent = (IControl)descendant.VisualParent;
             while (parent != null && parent != _owner)
             {
                 targetChild = parent;
@@ -362,26 +379,50 @@ namespace Avalonia.Controls
             return targetChild;
         }
 
-        public void ResetScrollers()
+        private void OnCompositionTargetRendering()
         {
-            _scroller = null;
-            _effectiveViewportChangedRevoker?.Dispose();
-            _effectiveViewportChangedRevoker = null;
-            _ensuredScroller = false;
+            _isBringIntoViewInProgress = false;
+            _makeAnchorElement = null;
+
+            if (_scroller is object)
+            {
+                foreach (var child in _owner.Children)
+                {
+                    var info = ItemsRepeater.GetVirtualizationInfo(child);
+
+                    if (info.IsRealized && info.IsHeldByLayout)
+                    {
+                        _scroller.RegisterAnchorCandidate(child);
+                    }
+                }
+            }
+
+            // HACK: Invalidate measure now that the anchor has been removed so that a layout can be
+            // done with a proper realization rect. This is a hack not present upstream to try to fix
+            // https://github.com/microsoft/microsoft-ui-xaml/issues/1422
+            TryInvalidateMeasure();
         }
 
-        private void OnEffectiveViewportChanged(TransformedBounds? bounds)
+        public void ResetScrollers()
         {
-            if (!bounds.HasValue)
+            if (_scroller is object)
             {
-                return;
+                foreach (var child in _owner.Children)
+                {
+                    _scroller.UnregisterAnchorCandidate(child);
+                }
+
+                _scroller = null;
             }
 
-            var globalClip = bounds.Value.Clip;
-            var transform = _owner.GetVisualRoot().TransformToVisual(_owner).Value;
-            var clip = globalClip.TransformToAABB(transform);
-            var effectiveViewport = clip.Intersect(bounds.Value.Bounds);
+            _effectiveViewportChangedRevoker?.Dispose();
+            _effectiveViewportChangedRevoker = null;
+            _ensuredScroller = false;
+        }
 
+        private void OnEffectiveViewportChanged(Rect effectiveViewport)
+        {
+            Logger.TryGet(LogEventLevel.Verbose, "Repeater")?.Log(this, "{LayoutId}: EffectiveViewportChanged event callback", _owner.Layout.LayoutId);
             UpdateViewport(effectiveViewport);
 
             _pendingViewportShift = default;
@@ -397,6 +438,7 @@ namespace Avalonia.Controls
             if (_layoutUpdatedSubscribed)
             {
                 _owner.LayoutUpdated -= OnLayoutUpdated;
+                _layoutUpdatedSubscribed = false;
             }
         }
 
@@ -437,10 +479,17 @@ namespace Avalonia.Controls
         private void UpdateViewport(Rect viewport)
         {
             var currentVisibleWindow = viewport;
+            var previousVisibleWindow = _visibleWindow;
+
+            Logger.TryGet(LogEventLevel.Verbose, "Repeater")?.Log(this, "{LayoutId}: Effective Viewport: ({Before})->({After})",
+                _owner.Layout.LayoutId,
+                previousVisibleWindow,
+                viewport);
 
             if (-currentVisibleWindow.X <= ItemsRepeater.ClearedElementsArrangePosition.X &&
                 -currentVisibleWindow.Y <= ItemsRepeater.ClearedElementsArrangePosition.Y)
             {
+                Logger.TryGet(LogEventLevel.Verbose, "Repeater")?.Log(this, "{LayoutId}: Viewport is invalid. visible window cleared", _owner.Layout.LayoutId);
                 // We got cleared.
                 _visibleWindow = default;
             }
@@ -449,7 +498,14 @@ namespace Avalonia.Controls
                 _visibleWindow = currentVisibleWindow;
             }
 
-            TryInvalidateMeasure();
+            if (_visibleWindow != previousVisibleWindow)
+            {
+                Logger.TryGet(LogEventLevel.Verbose, "Repeater")?.Log(this, "{LayoutId}: Used Viewport: ({Before})->({After})",
+                    _owner.Layout.LayoutId,
+                    previousVisibleWindow,
+                    currentVisibleWindow);
+                TryInvalidateMeasure();
+            }
         }
 
         private static void ValidateCacheLength(double cacheLength)
@@ -468,6 +524,7 @@ namespace Avalonia.Controls
                 // We invalidate measure instead of just invalidating arrange because
                 // we don't invalidate measure in UpdateViewport if the view is changing to
                 // avoid layout cycles.
+                Logger.TryGet(LogEventLevel.Verbose, "Repeater")?.Log(this, "{LayoutId}: Invalidating measure due to viewport change", _owner.Layout.LayoutId);
                 _owner.InvalidateMeasure();
             }
         }
@@ -476,16 +533,32 @@ namespace Avalonia.Controls
         {
             // HACK: This is a bit of a hack. We need the effective viewport of the ItemsRepeater -
             // we can get this from TransformedBounds, but this property is updated after layout has
-            // run, resulting in the UI being updated too late when scrolling quickly. We can
-            // partially remedey this by triggering also on Bounds changes, but this won't work so 
-            // well for nested ItemsRepeaters.
+            // run, which is too late. Instead, for now lets just hook into an internal event on
+            // ScrollContentPresenter to find out what the offset and viewport will be after arrange
+            // and use those values. Note that this doesn't handle nested ScrollViewers at all, but
+            // it's enough to get scrolling to non-uniformly sized items working for now.
             //
-            // UWP uses the EffectiveBoundsChanged event (which I think was implemented specially
-            // for this case): we need to implement that in Avalonia.
-            return control.GetObservable(Visual.TransformedBoundsProperty)
-                .Merge(control.GetObservable(Visual.BoundsProperty).Select(_ => control.TransformedBounds))
-                .Skip(1)
-                .Subscribe(OnEffectiveViewportChanged);
+            // UWP uses the EffectiveViewportChanged event (which I think was implemented specially
+            // for this case): we need to implement that in Avalonia, but the semantics of it aren't
+            // clear to me. Hopefully the source for this event will be released with WinUI 3.
+            if (control.VisualParent is ScrollContentPresenter scp)
+            {
+                scp.PreArrange += ScrollContentPresenterPreArrange;
+                return Disposable.Create(() => scp.PreArrange -= ScrollContentPresenterPreArrange);
+            }
+
+            return Disposable.Empty;
+        }
+
+        private void ScrollContentPresenterPreArrange(object sender, VectorEventArgs e)
+        {
+            var scp = (ScrollContentPresenter)sender;
+            var effectiveViewport = new Rect((Point)scp.Offset, new Size(e.Vector.X, e.Vector.Y));
+
+            if (effectiveViewport != _visibleWindow)
+            {
+                OnEffectiveViewportChanged(effectiveViewport);
+            }
         }
 
         private class ScrollerInfo

src/Avalonia.Controls/Repeater/VirtualizationInfo.cs

@@ -36,6 +36,7 @@ namespace Avalonia.Controls
         public bool IsHeldByLayout => Owner == ElementOwner.Layout;
         public bool IsRealized => IsHeldByLayout || Owner == ElementOwner.PinnedPool;
         public bool IsInUniqueIdResetPool => Owner == ElementOwner.UniqueIdResetPool;
+        public bool MustClearDataContext { get; set; }
         public bool KeepAlive { get; set; }
         public ElementOwner Owner { get; private set; } = ElementOwner.ElementFactory;
         public string UniqueId { get; private set; }

src/Avalonia.Controls/ScrollViewer.cs

@@ -1,4 +1,5 @@
 using System;
+using System.Security.Authentication.ExtendedProtection;
 using Avalonia.Controls.Presenters;
 using Avalonia.Controls.Primitives;
 using Avalonia.Input;
@@ -244,9 +245,7 @@ namespace Avalonia.Controls
 
             set
             {
-                value = ValidateOffset(this, value);
-
-                if (SetAndRaise(OffsetProperty, ref _offset, value))
+                if (SetAndRaise(OffsetProperty, ref _offset, CoerceOffset(Extent, Viewport, value)))
                 {
                     CalculatedPropertiesChanged();
                 }
@@ -316,6 +315,9 @@ namespace Avalonia.Controls
             get { return VerticalScrollBarVisibility != ScrollBarVisibility.Disabled; }
         }
 
+        /// <inheritdoc/>
+        public IControl CurrentAnchor => (Presenter as IScrollAnchorProvider)?.CurrentAnchor;
+
         /// <summary>
         /// Gets the maximum horizontal scrollbar value.
         /// </summary>
@@ -382,9 +384,6 @@ namespace Avalonia.Controls
             get { return _viewport.Height; }
         }
 
-        /// <inheritdoc/>
-        IControl IScrollAnchorProvider.CurrentAnchor => null; // TODO: Implement
-
         /// <summary>
         /// Scrolls the content up one line.
         /// </summary>
@@ -473,14 +472,16 @@ namespace Avalonia.Controls
             control.SetValue(VerticalScrollBarVisibilityProperty, value);
         }
 
-        void IScrollAnchorProvider.RegisterAnchorCandidate(IControl element)
+        /// <inheritdoc/>
+        public void RegisterAnchorCandidate(IControl element)
         {
-            // TODO: Implement
+            (Presenter as IScrollAnchorProvider)?.RegisterAnchorCandidate(element);
         }
 
-        void IScrollAnchorProvider.UnregisterAnchorCandidate(IControl element)
+        /// <inheritdoc/>
+        public void UnregisterAnchorCandidate(IControl element)
         {
-            // TODO: Implement
+            (Presenter as IScrollAnchorProvider)?.UnregisterAnchorCandidate(element);
         }
 
         protected override bool RegisterContentPresenter(IContentPresenter presenter)
@@ -517,22 +518,6 @@ namespace Avalonia.Controls
             return double.IsNaN(result) ? 0 : result;
         }
 
-        private static Vector ValidateOffset(AvaloniaObject o, Vector value)
-        {
-            ScrollViewer scrollViewer = o as ScrollViewer;
-
-            if (scrollViewer != null)
-            {
-                var extent = scrollViewer.Extent;
-                var viewport = scrollViewer.Viewport;
-                return CoerceOffset(extent, viewport, value);
-            }
-            else
-            {
-                return value;
-            }
-        }
-
         private void ChildChanged(IControl child)
         {
             if (_logicalScrollable is object)

src/Avalonia.Layout/AttachedLayout.cs

@@ -12,7 +12,7 @@ namespace Avalonia.Layout
     /// </summary>
     public abstract class AttachedLayout : AvaloniaObject
     {
-        internal string LayoutId { get; set; }
+        public string LayoutId { get; set; }
 
         /// <summary>
         /// Occurs when the measurement state (layout) has been invalidated.

src/Avalonia.Layout/ElementManager.cs

@@ -7,6 +7,7 @@ using System;
 using System.Collections.Generic;
 using System.Collections.Specialized;
 using Avalonia.Layout.Utils;
+using Avalonia.Logging;
 
 namespace Avalonia.Layout
 {
@@ -78,6 +79,7 @@ namespace Avalonia.Layout
                 {
                     // Sentinel. Create the element now since we need it.
                     int dataIndex = GetDataIndexFromRealizedRangeIndex(realizedIndex);
+                    Logger.TryGet(LogEventLevel.Verbose, "Repeater")?.Log(this, "Creating element for sentinal with data index {Index}", dataIndex);
                     element = _context.GetOrCreateElementAt(
                         dataIndex,
                         ElementRealizationOptions.ForceCreate | ElementRealizationOptions.SuppressAutoRecycle);
@@ -232,6 +234,8 @@ namespace Avalonia.Layout
                 {
                     Insert(0, dataIndex, element);
                 }
+
+                Logger.TryGet(LogEventLevel.Verbose, "Repeater")?.Log(this, "{LayoutId}: Created element for index {index}", layoutId, dataIndex);
             }
         }
 

src/Avalonia.Layout/FlowLayoutAlgorithm.cs

@@ -5,6 +5,7 @@
 
 using System;
 using System.Collections.Specialized;
+using Avalonia.Logging;
 
 namespace Avalonia.Layout
 {
@@ -82,6 +83,9 @@ namespace Avalonia.Layout
             // If minor size is infinity, there is only one line and no need to align that line.
             _scrollOrientationSameAsFlow = double.IsInfinity(_orientation.Minor(availableSize));
             var realizationRect = RealizationRect;
+            Logger.TryGet(LogEventLevel.Verbose, "Repeater")?.Log(this, "{LayoutId}: MeasureLayout Realization({Rect})",
+                layoutId,
+                realizationRect);
 
             var suggestedAnchorIndex = _context.RecommendedAnchorIndex;
             if (_elementManager.IsIndexValidInData(suggestedAnchorIndex))
@@ -100,6 +104,7 @@ namespace Avalonia.Layout
             Generate(GenerateDirection.Backward, anchorIndex, availableSize, minItemSpacing, lineSpacing, maxItemsPerLine, disableVirtualization, layoutId);
             if (isWrapping && IsReflowRequired())
             {
+                Logger.TryGet(LogEventLevel.Verbose, "Repeater")?.Log(this, "{LayoutId}: Reflow Pass", layoutId);
                 var firstElementBounds = _elementManager.GetLayoutBoundsForRealizedIndex(0);
                 _orientation.SetMinorStart(ref firstElementBounds, 0);
                 _elementManager.SetLayoutBoundsForRealizedIndex(0, firstElementBounds);
@@ -121,6 +126,7 @@ namespace Avalonia.Layout
             LineAlignment lineAlignment,
             string layoutId)
         {
+            Logger.TryGet(LogEventLevel.Verbose, "Repeater")?.Log(this, "{LayoutId}: ArrangeLayout", layoutId);
             ArrangeVirtualizingLayout(finalSize, lineAlignment, isWrapping, layoutId);
 
             return new Size(
@@ -184,6 +190,7 @@ namespace Avalonia.Layout
 
                 if (isAnchorSuggestionValid)
                 {
+                    Logger.TryGet(LogEventLevel.Verbose, "Repeater")?.Log(this, "{LayoutId}: Using suggested anchor {Anchor}", layoutId, suggestedAnchorIndex);
                     anchorIndex = _algorithmCallbacks.Algorithm_GetAnchorForTargetElement(
                         suggestedAnchorIndex,
                         availableSize,
@@ -223,6 +230,9 @@ namespace Avalonia.Layout
                 }
                 else if (needAnchorColumnRevaluation || !isRealizationWindowConnected)
                 {
+                    if (needAnchorColumnRevaluation) { Logger.TryGet(LogEventLevel.Verbose, "Repeater")?.Log(this, "{LayoutId}: NeedAnchorColumnReevaluation", layoutId); }
+                    if (!isRealizationWindowConnected) { Logger.TryGet(LogEventLevel.Verbose, "Repeater")?.Log(this, "{LayoutId}: Disconnected Window", layoutId); }
+
                     // The anchor is based on the realization window because a connected ItemsRepeater might intersect the realization window
                     // but not the visible window. In that situation, we still need to produce a valid anchor.
                     var anchorInfo = _algorithmCallbacks.Algorithm_GetAnchorForRealizationRect(availableSize, context);
@@ -231,6 +241,7 @@ namespace Avalonia.Layout
                 }
                 else
                 {
+                    Logger.TryGet(LogEventLevel.Verbose, "Repeater")?.Log(this, "{LayoutId}: Connected Window - picking first realized element as anchor", layoutId);
                     // No suggestion - just pick first in realized range
                     anchorIndex = _elementManager.GetDataIndexFromRealizedRangeIndex(0);
                     var firstElementBounds = _elementManager.GetLayoutBoundsForRealizedIndex(0);
@@ -238,12 +249,14 @@ namespace Avalonia.Layout
                 }
             }
 
+            Logger.TryGet(LogEventLevel.Verbose, "Repeater")?.Log(this, "{LayoutId}: Picked anchor: {Anchor}", layoutId, anchorIndex);
             _firstRealizedDataIndexInsideRealizationWindow = _lastRealizedDataIndexInsideRealizationWindow = anchorIndex;
             if (_elementManager.IsIndexValidInData(anchorIndex))
             {
                 if (!_elementManager.IsDataIndexRealized(anchorIndex))
                 {
                     // Disconnected, throw everything and create new anchor
+                    Logger.TryGet(LogEventLevel.Verbose, "Repeater")?.Log(this, "{LayoutId} Disconnected Window - throwing away all realized elements", layoutId);
                     _elementManager.ClearRealizedRange();
 
                     var anchor = _context.GetOrCreateElementAt(anchorIndex, ElementRealizationOptions.ForceCreate | ElementRealizationOptions.SuppressAutoRecycle);
@@ -254,9 +267,17 @@ namespace Avalonia.Layout
                 var desiredSize = MeasureElement(anchorElement, anchorIndex, availableSize, _context);
                 var layoutBounds = new Rect(anchorPosition.X, anchorPosition.Y, desiredSize.Width, desiredSize.Height);
                 _elementManager.SetLayoutBoundsForDataIndex(anchorIndex, layoutBounds);
+
+                Logger.TryGet(LogEventLevel.Verbose, "Repeater")?.Log(this, "{LayoutId}: Layout bounds of anchor {anchor} are ({Bounds})",
+                    layoutId,
+                    anchorIndex,
+                    layoutBounds);
             }
             else
             {
+                // Throw everything away
+                Logger.TryGet(LogEventLevel.Verbose, "Repeater")?.Log(this, "{LayoutId} Anchor index is not valid - throwing away all realized elements",
+                    layoutId);
                 _elementManager.ClearRealizedRange();
             }
 
@@ -280,6 +301,12 @@ namespace Avalonia.Layout
             if (anchorIndex != -1)
             {
                 int step = (direction == GenerateDirection.Forward) ? 1 : -1;
+
+                Logger.TryGet(LogEventLevel.Verbose, "Repeater")?.Log(this, "{LayoutId}: Generating {Direction} from anchor {Anchor}",
+                    layoutId,
+                    direction,
+                    anchorIndex);
+
                 int previousIndex = anchorIndex;
                 int currentIndex = anchorIndex + step;
                 var anchorBounds = _elementManager.GetLayoutBoundsForDataIndex(anchorIndex);
@@ -365,6 +392,10 @@ namespace Avalonia.Layout
                                         _orientation.SetMajorStart(ref bounds, previousLineOffset - lineMajorSize - lineSpacing);
                                         _orientation.SetMajorSize(ref bounds, lineMajorSize);
                                         _elementManager.SetLayoutBoundsForDataIndex(dataIndex, bounds);
+                                        Logger.TryGet(LogEventLevel.Verbose, "Repeater")?.Log(this, "{LayoutId}: Corrected Layout bounds of element {Index} are ({Bounds})",
+                                            layoutId,
+                                            dataIndex,
+                                            bounds);
                                     }
                                 }
                             }
@@ -387,6 +418,11 @@ namespace Avalonia.Layout
                     }
 
                     _elementManager.SetLayoutBoundsForDataIndex(currentIndex, currentBounds);
+
+                    Logger.TryGet(LogEventLevel.Verbose, "Repeater")?.Log(this, "{LayoutId}: Layout bounds of element {Index} are ({Bounds}).",
+                        layoutId,
+                        currentIndex,
+                        currentBounds);
                     previousIndex = currentIndex;
                     currentIndex += step;
                 }
@@ -394,20 +430,17 @@ namespace Avalonia.Layout
                 // If we did not reach the top or bottom of the extent, we realized one 
                 // extra item before we knew we were outside the realization window. Do not
                 // account for that element in the indicies inside the realization window.
-                if (count > 0)
+                if (direction == GenerateDirection.Forward)
                 {
-                    if (direction == GenerateDirection.Forward)
-                    {
-                        int dataCount = _context.ItemCount;
-                        _lastRealizedDataIndexInsideRealizationWindow = previousIndex == dataCount - 1 ? dataCount - 1 : previousIndex - 1;
-                        _lastRealizedDataIndexInsideRealizationWindow = Math.Max(0, _lastRealizedDataIndexInsideRealizationWindow);
-                    }
-                    else
-                    {
-                        int dataCount = _context.ItemCount;
-                        _firstRealizedDataIndexInsideRealizationWindow = previousIndex == 0 ? 0 : previousIndex + 1;
-                        _firstRealizedDataIndexInsideRealizationWindow = Math.Min(dataCount - 1, _firstRealizedDataIndexInsideRealizationWindow);
-                    }
+                    int dataCount = _context.ItemCount;
+                    _lastRealizedDataIndexInsideRealizationWindow = previousIndex == dataCount - 1 ? dataCount - 1 : previousIndex - 1;
+                    _lastRealizedDataIndexInsideRealizationWindow = Math.Max(0, _lastRealizedDataIndexInsideRealizationWindow);
+                }
+                else
+                {
+                    int dataCount = _context.ItemCount;
+                    _firstRealizedDataIndexInsideRealizationWindow = previousIndex == 0 ? 0 : previousIndex + 1;
+                    _firstRealizedDataIndexInsideRealizationWindow = Math.Min(dataCount - 1, _firstRealizedDataIndexInsideRealizationWindow);
                 }
 
                 _elementManager.DiscardElementsOutsideWindow(direction == GenerateDirection.Forward, currentIndex);
@@ -508,6 +541,7 @@ namespace Avalonia.Layout
                 lastDataIndex,
                 lastBounds);
 
+            Logger.TryGet(LogEventLevel.Verbose, "Repeater")?.Log(this, "{LayoutId} Extent: ({Bounds})", layoutId, extent);
             return extent;
         }
 
@@ -676,6 +710,11 @@ namespace Avalonia.Layout
                 }
 
                 var element = _elementManager.GetAt(rangeIndex);
+
+                Logger.TryGet(LogEventLevel.Verbose, "Repeater")?.Log(this, "{LayoutId}: Arranging element {Index} at ({Bounds})",
+                    layoutId,
+                    _elementManager.GetDataIndexFromRealizedRangeIndex(rangeIndex),
+                    bounds);
                 element.Arrange(bounds);
             }
         }

src/Avalonia.Layout/LayoutQueue.cs

@@ -77,7 +77,7 @@ namespace Avalonia.Layout
             {
                 if (_shouldEnqueue(item.Key))
                 {
-                    _loopQueueInfo[item.Key] = new Info() { Active = true, Count = item.Value.Count + 1 };
+                    _loopQueueInfo[item.Key] = new Info() { Active = true, Count = 0 };
                     _inner.Enqueue(item.Key);
                 }
             }

src/Avalonia.Layout/NonVirtualizingStackLayout.cs

@@ -112,7 +112,9 @@ namespace Avalonia.Layout
                 u = (isVertical ? bounds.Bottom : bounds.Right) + spacing;
             }
 
-            return new Size(bounds.Right, bounds.Bottom);
+            return new Size(
+                Math.Max(finalSize.Width, bounds.Width),
+                Math.Max(finalSize.Height, bounds.Height));
         }
 
         private static Rect LayoutVertical(ILayoutable element, double y, Size constraint)

src/Avalonia.Layout/StackLayout.cs

@@ -6,6 +6,7 @@
 using System;
 using System.Collections.Specialized;
 using Avalonia.Data;
+using Avalonia.Logging;
 
 namespace Avalonia.Layout
 {
@@ -107,8 +108,15 @@ namespace Avalonia.Layout
                             _orientation.MajorStart(extent) + 
                             (remainingItems * averageElementSize));
                 }
+                else
+                {
+                    Logger.TryGet(LogEventLevel.Verbose, "Repeater")?.Log(this, "{LayoutId}: Estimating extent with no realized elements",
+                        LayoutId);
+                }
             }
 
+            Logger.TryGet(LogEventLevel.Verbose, "Repeater")?.Log(this, "{LayoutId}: Extent is ({Size}). Based on average {Average}",
+                LayoutId, extent.Size, averageElementSize);
             return extent;
         }
 

src/Avalonia.Layout/UniformGridLayout.cs

@@ -6,6 +6,7 @@
 using System;
 using System.Collections.Specialized;
 using Avalonia.Data;
+using Avalonia.Logging;
 
 namespace Avalonia.Layout
 {
@@ -379,8 +380,14 @@ namespace Avalonia.Layout
                         ref extent,
                         _orientation.MajorEnd(lastRealizedLayoutBounds) - _orientation.MajorStart(extent) + (remainingItems / itemsPerLine) * lineSize);
                 }
+                else
+                {
+                    Logger.TryGet(LogEventLevel.Verbose, "Repeater")?.Log(this, "{LayoutId}: Estimating extent with no realized elements", LayoutId);
+                }
             }
 
+            Logger.TryGet(LogEventLevel.Verbose, "Repeater")?.Log(this, "{LayoutId}: Extent is ({Size}). Based on lineSize {LineSize} and items per line {ItemsPerLine}",
+                LayoutId, extent.Size, lineSize, itemsPerLine);
             return extent;
         }
 

tests/Avalonia.Controls.UnitTests/Presenters/ScrollContentPresenterTests.cs

@@ -117,13 +117,17 @@ namespace Avalonia.Controls.UnitTests.Presenters
                     Width = 150,
                     Height = 150,
                 },
-                Offset = new Vector(25, 25),
             };
 
             target.UpdateChild();
             target.Measure(new Size(100, 100));
             target.Arrange(new Rect(0, 0, 100, 100));
 
+            target.Offset = new Vector(25, 25);
+
+            target.Measure(new Size(100, 100));
+            target.Arrange(new Rect(0, 0, 100, 100));
+
             Assert.Equal(new Rect(-25, -25, 150, 150), content.Bounds);
         }
 
@@ -345,4 +349,4 @@ namespace Avalonia.Controls.UnitTests.Presenters
             }
         }
     }
-}
+}

tests/Avalonia.Controls.UnitTests/Presenters/ScrollContentPresenterTests_ILogicalScrollable.cs

@@ -59,13 +59,17 @@ namespace Avalonia.Controls.UnitTests
                 CanHorizontallyScroll = true,
                 CanVerticallyScroll = true,
                 Content = scrollable,
-                Offset = new Vector(25, 25),
             };
 
             target.UpdateChild();
             target.Measure(new Size(100, 100));
             target.Arrange(new Rect(0, 0, 100, 100));
 
+            target.Offset = new Vector(25, 25);
+            
+            target.Measure(new Size(100, 100));
+            target.Arrange(new Rect(0, 0, 100, 100));
+
             Assert.Equal(new Rect(-25, -25, 150, 150), scrollable.Bounds);
         }