Don't register anchor candidates until they're laid out.

src/Avalonia.Controls/Repeater/ItemsRepeater.cs

@@ -369,6 +369,12 @@ namespace Avalonia.Controls
                     {
                         var newBounds = element.Bounds;
                         virtInfo.ArrangeBounds = newBounds;
+
+                        if (!virtInfo.IsRegisteredAsAnchorCandidate)
+                        {
+                            _viewportManager.RegisterScrollAnchorCandidate(element);
+                            virtInfo.IsRegisteredAsAnchorCandidate = true;
+                        }
                     }
                 }
 
@@ -520,11 +526,14 @@ namespace Avalonia.Controls
             return element;
         }
 
-        internal void OnElementPrepared(IControl element, int index)
+        internal void OnElementPrepared(IControl element, VirtualizationInfo virtInfo)
         {
-            _viewportManager.OnElementPrepared(element);
+            _viewportManager.OnElementPrepared(element, virtInfo);
+
             if (ElementPrepared != null)
             {
+                var index = virtInfo.Index;
+
                 if (_elementPreparedArgs == null)
                 {
                     _elementPreparedArgs = new ItemsRepeaterElementPreparedEventArgs(element, index);

src/Avalonia.Controls/Repeater/ViewManager.cs

@@ -661,7 +661,7 @@ namespace Avalonia.Controls
                 children.Add(element);
             }
 
-            repeater.OnElementPrepared(element, index);
+            repeater.OnElementPrepared(element, virtInfo);
 
             // Update realized indices
             _firstRealizedElementIndexHeldByLayout = Math.Min(_firstRealizedElementIndexHeldByLayout, index);

src/Avalonia.Controls/Repeater/ViewportManager.cs

@@ -240,9 +240,14 @@ namespace Avalonia.Controls
             }
         }
 
-        public void OnElementPrepared(IControl element)
+        public void OnElementPrepared(IControl element, VirtualizationInfo virtInfo)
         {
-            _scroller?.RegisterAnchorCandidate(element);
+            // WinUI registers the element as an anchor candidate here, but I feel that's in error:
+            // at this point the element has not yet been positioned by the arrange pass so it will
+            // have its previous position, meaning that when the arrange pass moves it into its new
+            // position, an incorrect scroll anchoring will occur. Instead signal that it's not yet
+            // registered as a scroll anchor candidate.
+            virtInfo.IsRegisteredAsAnchorCandidate = false;
         }
 
         public void OnElementCleared(IControl element)
@@ -373,6 +378,11 @@ namespace Avalonia.Controls
             }
         }
 
+        public void RegisterScrollAnchorCandidate(IControl element)
+        {
+            _scroller?.RegisterAnchorCandidate(element);
+        }
+
         private IControl GetImmediateChildOfRepeater(IControl descendant)
         {
             var targetChild = descendant;

src/Avalonia.Controls/Repeater/VirtualizationInfo.cs

@@ -38,6 +38,7 @@ namespace Avalonia.Controls
         public bool IsInUniqueIdResetPool => Owner == ElementOwner.UniqueIdResetPool;
         public bool MustClearDataContext { get; set; }
         public bool KeepAlive { get; set; }
+        public bool IsRegisteredAsAnchorCandidate { get; set; }
         public ElementOwner Owner { get; private set; } = ElementOwner.ElementFactory;
         public string UniqueId { get; private set; }