Tweak ScrollContentPresenter anchoring.

Using WinUI's `ScrollPresenter` implementation to guide us.

src/Avalonia.Controls/Presenters/ScrollContentPresenter.cs

@@ -16,6 +16,8 @@ namespace Avalonia.Controls.Presenters
     /// </summary>
     public class ScrollContentPresenter : ContentPresenter, IPresenter, IScrollable, IScrollAnchorProvider
     {
+        private const double EdgeDetectionTolerance = 0.1;
+
         /// <summary>
         /// Defines the <see cref="CanHorizontallyScroll"/> property.
         /// </summary>
@@ -70,7 +72,9 @@ namespace Avalonia.Controls.Presenters
         private Size _viewport;
         private Dictionary<int, Vector>? _activeLogicalGestureScrolls;
         private List<IControl>? _anchorCandidates;
-        private (IControl? control, Rect bounds) _anchor;
+        private IControl? _anchorElement;
+        private Rect _anchorElementBounds;
+        private bool _isAnchorElementDirty;
 
         /// <summary>
         /// Initializes static members of the <see cref="ScrollContentPresenter"/> class.
@@ -138,7 +142,14 @@ namespace Avalonia.Controls.Presenters
         }
 
         /// <inheritdoc/>
-        IControl? IScrollAnchorProvider.CurrentAnchor => _anchor.control;
+        IControl? IScrollAnchorProvider.CurrentAnchor
+        {
+            get
+            {
+                EnsureAnchorElementSelection();
+                return _anchorElement;
+            }
+        }
 
         /// <summary>
         /// Attempts to bring a portion of the target visual into view by scrolling the content.
@@ -215,16 +226,18 @@ namespace Avalonia.Controls.Presenters
 
             _anchorCandidates ??= new List<IControl>();
             _anchorCandidates.Add(element);
+            _isAnchorElementDirty = true;
         }
 
         /// <inheritdoc/>
         void IScrollAnchorProvider.UnregisterAnchorCandidate(IControl element)
         {
             _anchorCandidates?.Remove(element);
+            _isAnchorElementDirty = true;
 
-            if (_anchor.control == element)
+            if (_anchorElement == element)
             {
-                _anchor = default;
+                _anchorElement = null;
             }
         }
 
@@ -266,59 +279,69 @@ namespace Avalonia.Controls.Presenters
                 // 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 &&
+                if (_anchorElement != null &&
-                    TranslateBounds(_anchor.control, Child, out var updatedBounds) &&
+                    TranslateBounds(_anchorElement, Child, out var updatedBounds) &&
-                    updatedBounds.Position != _anchor.bounds.Position)
+                    updatedBounds.Position != _anchorElementBounds.Position)
                 {
-                    var offset = updatedBounds.Position - _anchor.bounds.Position;
+                    var offset = updatedBounds.Position - _anchorElementBounds.Position;
                     return offset;
                 }
 
                 return default;
             }
 
-            // Calculate the new anchor element.
+            var isAnchoring = Offset.X >= EdgeDetectionTolerance || Offset.Y >= EdgeDetectionTolerance;
-            _anchor = CalculateCurrentAnchor();
 
-            // Do the arrange.
+            if (isAnchoring)
-            ArrangeOverrideImpl(size, -Offset);
+            {
+                // Calculate the new anchor element if necessary.
+                EnsureAnchorElementSelection();
 
-            // If the anchor moved during the arrange, we need to adjust the offset and do another arrange.
+                // Do the arrange.
-            var anchorShift = TrackAnchor();
+                ArrangeOverrideImpl(size, -Offset);
 
-            if (anchorShift != default)
+                // If the anchor moved during the arrange, we need to adjust the offset and do another arrange.
-            {
+                var anchorShift = TrackAnchor();
-                var newOffset = Offset + anchorShift;
-                var newExtent = Extent;
-                var maxOffset = new Vector(Extent.Width - Viewport.Width, Extent.Height - Viewport.Height);
 
-                if (newOffset.X > maxOffset.X)
+                if (anchorShift != default)
                 {
-                    newExtent = newExtent.WithWidth(newOffset.X + Viewport.Width);
+                    var newOffset = Offset + anchorShift;
-                }
+                    var newExtent = Extent;
+                    var maxOffset = new Vector(Extent.Width - Viewport.Width, Extent.Height - Viewport.Height);
 
-                if (newOffset.Y > maxOffset.Y)
+                    if (newOffset.X > maxOffset.X)
-                {
+                    {
-                    newExtent = newExtent.WithHeight(newOffset.Y + Viewport.Height);
+                        newExtent = newExtent.WithWidth(newOffset.X + Viewport.Width);
-                }
+                    }
 
-                Extent = newExtent;
+                    if (newOffset.Y > maxOffset.Y)
+                    {
+                        newExtent = newExtent.WithHeight(newOffset.Y + Viewport.Height);
+                    }
 
-                try
+                    Extent = newExtent;
-                {
+
-                    _arranging = true;
+                    try
-                    Offset = newOffset;
+                    {
-                }
+                        _arranging = true;
-                finally
+                        Offset = newOffset;
-                {
+                    }
-                    _arranging = false;    
+                    finally
+                    {
+                        _arranging = false;
+                    }
+
+                    ArrangeOverrideImpl(size, -Offset);
                 }
-                
+            }
+            else
+            {
                 ArrangeOverrideImpl(size, -Offset);
             }
 
             Viewport = finalSize;
             Extent = Child.Bounds.Size.Inflate(Child.Margin);
+            _isAnchorElementDirty = true;
 
             return finalSize;
         }
@@ -493,13 +516,17 @@ namespace Avalonia.Controls.Presenters
             }
         }
 
-        private (IControl, Rect) CalculateCurrentAnchor()
+        private void EnsureAnchorElementSelection()
         {
-            if (_anchorCandidates == null)
+            if (!_isAnchorElementDirty || _anchorCandidates is null)
             {
-                return default;
+                return;
             }
 
+            _anchorElement = null;
+            _anchorElementBounds = default;
+            _isAnchorElementDirty = false;
+
             var bestCandidate = default(IControl);
             var bestCandidateDistance = double.MaxValue;
 
@@ -526,10 +553,9 @@ namespace Avalonia.Controls.Presenters
                 // 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);
+                _anchorElement = bestCandidate;
+                _anchorElementBounds = unscrolledBounds;
             }
-
-            return default;
         }
 
         private bool GetViewportBounds(IControl element, out Rect bounds)