Fix for remaining Ghost-Item in VirtualizationStackPanel (#20784)

* Add failing test for #20688

Unit test created from issue description

* Add a failing test for wrong CacheLength handling

All items would be realized which is obviously wrong

* fix test setup used StackPanel instead of expcected

VirtualizingStackPanel

* Make sure the test actually fails

* update comment

* Fix for focusedElement and focusedIndex

* add another unit test

* Fixes for new test cases

* Addressing Review

* Update tests to match new behavior

* only recycle focused element if it is not null

* Address review

* Address copilot review

* add failing test

* fix StartU estimation

* remove unused sample file

src/Avalonia.Controls/VirtualizingStackPanel.cs

@@ -858,7 +858,32 @@ namespace Avalonia.Controls
             // Estimate the element size.
             var estimatedSize = EstimateElementSizeU();
 
-            // TODO: Use _startU to work this out.
+            // If we have a valid StartU, use it to anchor estimates relative to the realized range.
+            if (_realizedElements is { } realized && !double.IsNaN(realized.StartU))
+            {
+                var first = realized.FirstIndex;
+                var last = realized.LastIndex;
+            
+                if (index < first)
+                {
+                    return realized.StartU - ((first - index) * estimatedSize);
+                }
+            
+                if (index > last)
+                {
+                    var sizes = realized.SizeU;
+                    var realizedSpan = 0.0;
+            
+                    for (var i = 0; i < sizes.Count; ++i)
+                    {
+                        var sizeU = sizes[i];
+                        realizedSpan += double.IsNaN(sizeU) ? estimatedSize : sizeU;
+                    }
+            
+                    return realized.StartU + realizedSpan + ((index - last - 1) * estimatedSize);
+                }
+            }
+
             return index * estimatedSize;
         }
 

tests/Avalonia.Controls.UnitTests/VirtualizingStackPanelTests.cs

@@ -1553,6 +1553,40 @@ namespace Avalonia.Controls.UnitTests
             Assert.Equal(new Rect(0, 125, 100, 25), container.Bounds);
         }
 
+        [Fact]
+        public void Focused_Container_Is_Positioned_Correctly_When_Scrolled_Past_Items_With_Different_Heights()
+        {
+            using var app = App();
+
+            var items = Enumerable.Range(0, 20)
+                .Select(x => new ItemWithHeight(x, x < 10 ? 10 : 50))
+                .ToList();
+
+            var (target, _, _) = CreateTarget(items: items, itemTemplate: CanvasWithHeightTemplate);
+
+            var focused = Assert.IsType<ContentPresenter>(target.ContainerFromIndex(5));
+            focused.Focusable = true;
+            focused.Focus();
+
+            target.ScrollIntoView(15);
+            Layout(target);
+
+            Assert.True(target.FirstRealizedIndex > 5);
+
+            var firstIndex = target.FirstRealizedIndex;
+            var firstRealized = Assert.IsType<ContentPresenter>(target.ContainerFromIndex(firstIndex));
+            var realized = target.GetRealizedElements()
+                .Where(x => x is not null)
+                .Cast<Control>()
+                .ToList();
+
+            var estimatedSize = realized.Average(x => x.DesiredSize.Height);
+            var expectedTop = firstRealized.Bounds.Top - ((firstIndex - 5) * estimatedSize);
+
+            focused = Assert.IsType<ContentPresenter>(target.ContainerFromIndex(5));
+            Assert.Equal(expectedTop, focused.Bounds.Top, 3);
+        }
+
         [Theory]
         [InlineData(0d, 
             4, 7,