From b630f21b7e8c940f51fa341d575027041b92bd96 Mon Sep 17 00:00:00 2001
From: Steven Kirk <grokys@users.noreply.github.com>
Date: Tue, 28 Nov 2023 22:40:37 +0100
Subject: [PATCH] Fix overlapping items in `VirtualizingStackPanel` (#13765)

* Added failing test for #12744.

* Inserting items makes the StartU unstable.

This ports the fix in https://github.com/AvaloniaUI/Avalonia.Controls.TreeDataGrid/pull/229 to Avalonia.

Fixes #12744
---
 .../Utils/RealizedStackElements.cs            |  1 +
 .../VirtualizingStackPanelTests.cs            | 22 +++++++++++++++++++
 2 files changed, 23 insertions(+)

diff --git a/src/Avalonia.Controls/Utils/RealizedStackElements.cs b/src/Avalonia.Controls/Utils/RealizedStackElements.cs
index 11bbaa11c4..18cba5b123 100644
--- a/src/Avalonia.Controls/Utils/RealizedStackElements.cs
+++ b/src/Avalonia.Controls/Utils/RealizedStackElements.cs
@@ -294,6 +294,7 @@ namespace Avalonia.Controls.Utils
                 {
                     // The insertion point was before the first element, update the first index.
                     _firstIndex += count;
+                    _startUUnstable = true;
                 }
                 else
                 {
diff --git a/tests/Avalonia.Controls.UnitTests/VirtualizingStackPanelTests.cs b/tests/Avalonia.Controls.UnitTests/VirtualizingStackPanelTests.cs
index 4cbf8d2142..d0e139bedf 100644
--- a/tests/Avalonia.Controls.UnitTests/VirtualizingStackPanelTests.cs
+++ b/tests/Avalonia.Controls.UnitTests/VirtualizingStackPanelTests.cs
@@ -843,6 +843,28 @@ namespace Avalonia.Controls.UnitTests
             Assert.Equal(1, panel.VisualChildren.Count);
         }
 
+        [Fact]
+        public void Inserting_Item_Before_Viewport_Preserves_FirstRealizedIndex()
+        {
+            // Issue #12744
+            using var app = App();
+            var (target, scroll, itemsControl) = CreateTarget();
+            var items = (IList)itemsControl.ItemsSource!;
+
+            // Scroll down 20 items.
+            scroll.Offset = new Vector(0, 200);
+            target.UpdateLayout();
+            Assert.Equal(20, target.FirstRealizedIndex);
+
+            // Insert an item at the beginning.
+            items.Insert(0, "New Item");
+            target.UpdateLayout();
+
+            // The first realized index should still be 20 as the scroll should be unchanged.
+            Assert.Equal(20, target.FirstRealizedIndex);
+            Assert.Equal(new(0, 200), scroll.Offset);
+        }
+
         private static IReadOnlyList<int> GetRealizedIndexes(VirtualizingStackPanel target, ItemsControl itemsControl)
         {
             return target.GetRealizedElements()