Allow virtualized list to take up available space.

src/Avalonia.Controls/VirtualizingStackPanel.cs

@@ -3,6 +3,7 @@
 
 using System;
 using System.Collections.Specialized;
+using Avalonia.Layout;
 
 namespace Avalonia.Controls
 {
@@ -19,8 +20,8 @@ namespace Avalonia.Controls
             get
             {
                 return Orientation == Orientation.Horizontal ?
-                    _takenSpace >= Bounds.Width :
-                    _takenSpace >= Bounds.Height;
+                    _takenSpace >= AvailableSpace.Width :
+                    _takenSpace >= AvailableSpace.Height;
             }
         }
 
@@ -54,6 +55,9 @@ namespace Avalonia.Controls
             }
         }
 
+        // TODO: We need to put a reasonable limit on this, probably based on the max window size.
+        private Size AvailableSpace => ((ILayoutable)this).PreviousMeasure ?? Bounds.Size;
+
         protected override Size ArrangeOverride(Size finalSize)
         {
             _canBeRemoved = 0;
@@ -99,7 +103,7 @@ namespace Avalonia.Controls
                 rect = new Rect(rect.X, rect.Y - _pixelOffset, rect.Width, rect.Height);
                 child.Arrange(rect);
 
-                if (rect.Y >= panelSize.Height)
+                if (rect.Y >= AvailableSpace.Height)
                 {
                     ++_canBeRemoved;
                 }
@@ -116,7 +120,7 @@ namespace Avalonia.Controls
                 rect = new Rect(rect.X - _pixelOffset, rect.Y, rect.Width, rect.Height);
                 child.Arrange(rect);
 
-                if (rect.X >= panelSize.Width)
+                if (rect.X >= AvailableSpace.Width)
                 {
                     ++_canBeRemoved;
                 }
@@ -135,7 +139,7 @@ namespace Avalonia.Controls
             var bounds = Bounds;
             var gap = Gap;
 
-            child.Measure(bounds.Size);
+            child.Measure(AvailableSpace);
             ++_averageCount;
 
             if (Orientation == Orientation.Vertical)

tests/Avalonia.Controls.UnitTests/Avalonia.Controls.UnitTests.csproj

@@ -143,6 +143,7 @@
     <Compile Include="TestTemplatedControl.cs" />
     <Compile Include="Utils\AncestorFinderTests.cs" />
     <Compile Include="Utils\HotKeyManagerTests.cs" />
+    <Compile Include="VirtualizingStackPanelTests.cs" />
     <Compile Include="WrapPanelTests.cs" />
   </ItemGroup>
   <ItemGroup>

tests/Avalonia.Controls.UnitTests/Presenters/ItemsPresenterTests_Virtualization.cs

@@ -111,6 +111,7 @@ namespace Avalonia.Controls.UnitTests.Presenters
 
             Assert.Equal(10, target.Panel.Children.Count);
 
+            target.Measure(new Size(120, 120));
             target.Arrange(new Rect(0, 0, 100, 120));
 
             Assert.Equal(12, target.Panel.Children.Count);

tests/Avalonia.Controls.UnitTests/Presenters/ItemsPresenterTests_Virtualization_Simple.cs

@@ -72,6 +72,7 @@ namespace Avalonia.Controls.UnitTests.Presenters
 
             Assert.Equal(10, target.Panel.Children.Count);
 
+            target.Measure(new Size(100, 80));
             target.Arrange(new Rect(0, 0, 100, 80));
 
             Assert.Equal(8, target.Panel.Children.Count);
@@ -90,6 +91,7 @@ namespace Avalonia.Controls.UnitTests.Presenters
             Assert.Equal(10, target.Panel.Children.Count);
 
             ((IScrollable)target).Offset = new Vector(0, 10);
+            target.Measure(new Size(120, 120));
             target.Arrange(new Rect(0, 0, 100, 120));
 
             Assert.Equal(12, target.Panel.Children.Count);

tests/Avalonia.Controls.UnitTests/VirtualizingStackPanelTests.cs

@@ -0,0 +1,52 @@
+// Copyright (c) The Avalonia Project. All rights reserved.
+// Licensed under the MIT license. See licence.md file in the project root for full license information.
+
+using Xunit;
+
+namespace Avalonia.Controls.UnitTests
+{
+    public class VirtualizingStackPanelTests
+    {
+        public class Vertical
+        {
+            [Fact]
+            public void Reports_IsFull_False_Until_Measure_Height_Is_Reached()
+            {
+                var target = (IVirtualizingPanel)new VirtualizingStackPanel();
+
+                target.Measure(new Size(100, 100));
+                target.Arrange(new Rect(target.DesiredSize));
+
+                Assert.Equal(new Size(0, 0), target.Bounds.Size);
+
+                Assert.False(target.IsFull);
+                target.Children.Add(new Canvas { Width = 50, Height = 50 });
+                Assert.False(target.IsFull);
+                target.Children.Add(new Canvas { Width = 50, Height = 50 });
+                Assert.True(target.IsFull);
+            }
+
+            [Fact]
+            public void Reports_Overflow_Only_After_Arrange()
+            {
+                var target = (IVirtualizingPanel)new VirtualizingStackPanel();
+
+                target.Measure(new Size(100, 100));
+                target.Arrange(new Rect(target.DesiredSize));
+
+                Assert.Equal(new Size(0, 0), target.Bounds.Size);
+
+                target.Children.Add(new Canvas { Width = 50, Height = 50 });
+                target.Children.Add(new Canvas { Width = 50, Height = 50 });
+                target.Children.Add(new Canvas { Width = 50, Height = 50 });
+                target.Children.Add(new Canvas { Width = 50, Height = 50 });
+                Assert.Equal(0, target.OverflowCount);
+
+                target.Measure(new Size(100, 100));
+                target.Arrange(new Rect(target.DesiredSize));
+
+                Assert.Equal(2, target.OverflowCount);
+            }
+        }
+    }
+}