From 9f95bd9156a64b7a2af376cb98947895234bb867 Mon Sep 17 00:00:00 2001
From: Steven Kirk <grokys@users.noreply.github.com>
Date: Mon, 3 Apr 2023 18:07:27 +0200
Subject: [PATCH] Recycle elements early when viewport is disjunct.

---
 .../VirtualizingStackPanel.cs                 | 11 +++++++
 .../VirtualizingStackPanelTests.cs            | 31 ++++++++++++++++++-
 2 files changed, 41 insertions(+), 1 deletion(-)

diff --git a/src/Avalonia.Controls/VirtualizingStackPanel.cs b/src/Avalonia.Controls/VirtualizingStackPanel.cs
index 6045f85135..850511f5d1 100644
--- a/src/Avalonia.Controls/VirtualizingStackPanel.cs
+++ b/src/Avalonia.Controls/VirtualizingStackPanel.cs
@@ -166,6 +166,10 @@ namespace Avalonia.Controls
                 // - Vertical layouts: U = vertical, V = horizontal
                 var viewport = CalculateMeasureViewport(items);
 
+                // If the viewport is disjunct then we can recycle everything.
+                if (viewport.viewportIsDisjunct)
+                    _realizedElements.RecycleAllElements(_recycleElement);
+
                 // Do the measure, creating/recycling elements as necessary to fill the viewport. Don't
                 // write to _realizedElements yet, only _measureElements.
                 RealizeElements(items, availableSize, ref viewport);
@@ -409,6 +413,7 @@ namespace Avalonia.Controls
             var viewportStart = Orientation == Orientation.Horizontal ? viewport.X : viewport.Y;
             var viewportEnd = Orientation == Orientation.Horizontal ? viewport.Right : viewport.Bottom;
 
+            // Get or estimate the anchor element from which to start realization.
             var itemCount = items?.Count ?? 0;
             var (anchorIndex, anchorU) = _realizedElements.GetOrEstimateAnchorElementForViewport(
                 viewportStart,
@@ -416,12 +421,17 @@ namespace Avalonia.Controls
                 itemCount,
                 ref _lastEstimatedElementSizeU);
 
+            // Check if the anchor element is not within the currently realized elements.
+            var disjunct = anchorIndex < _realizedElements.FirstIndex || 
+                anchorIndex > _realizedElements.LastIndex;
+
             return new MeasureViewport
             {
                 anchorIndex = anchorIndex,
                 anchorU = anchorU,
                 viewportUStart = viewportStart,
                 viewportUEnd = viewportEnd,
+                viewportIsDisjunct = disjunct,
             };
         }
 
@@ -895,6 +905,7 @@ namespace Avalonia.Controls
             public double measuredV;
             public double realizedEndU;
             public int lastIndex;
+            public bool viewportIsDisjunct;
         }
     }
 }
diff --git a/tests/Avalonia.Controls.UnitTests/VirtualizingStackPanelTests.cs b/tests/Avalonia.Controls.UnitTests/VirtualizingStackPanelTests.cs
index 2cdd4eaf95..e8b4b8c513 100644
--- a/tests/Avalonia.Controls.UnitTests/VirtualizingStackPanelTests.cs
+++ b/tests/Avalonia.Controls.UnitTests/VirtualizingStackPanelTests.cs
@@ -83,7 +83,7 @@ namespace Avalonia.Controls.UnitTests
         }
 
         [Fact]
-        public void Scrolls_To_Index()
+        public void Scrolls_Down_To_Index()
         {
             using var app = App();
             var (target, scroll, itemsControl) = CreateTarget();
@@ -93,6 +93,35 @@ namespace Avalonia.Controls.UnitTests
             AssertRealizedItems(target, itemsControl, 11, 10);
         }
 
+        [Fact]
+        public void Scrolls_Up_To_Index()
+        {
+            using var app = App();
+            var (target, scroll, itemsControl) = CreateTarget();
+
+            scroll.ScrollToEnd();
+            Layout(target);
+
+            Assert.Equal(90, target.FirstRealizedIndex);
+
+            target.ScrollIntoView(20);
+
+            AssertRealizedItems(target, itemsControl, 20, 10);
+        }
+
+        [Fact]
+        public void Scrolling_Up_To_Index_Does_Not_Create_A_Page_Of_Unrealized_Elements()
+        {
+            using var app = App();
+            var (target, scroll, itemsControl) = CreateTarget();
+
+            scroll.ScrollToEnd();
+            Layout(target);
+            target.ScrollIntoView(20);
+
+            Assert.Equal(11, target.Children.Count);
+        }
+
         [Fact]
         public void Creates_Elements_On_Item_Insert_1()
         {