Use NonVirtualizingLayoutContext.

- Add and use `NonVirtualizingLayoutContext` in `NonVirtualizingLayout`
- Return -1 instead of throwing if element is not a child of `ItemsRepeater`

Ported from microsoft/microsoft-ui-xaml@22adf4d

src/Avalonia.Controls/Repeater/ViewManager.cs

@@ -200,7 +200,8 @@ public int GetElementIndex(VirtualizationInfo virtInfo)
         {
             if (virtInfo == null)
             {
-                throw new ArgumentException("Element is not a child of this ItemsRepeater.");
+                //Element is not a child of this ItemsRepeater.
+                return -1;
             }
 
             return virtInfo.IsRealized || virtInfo.IsInUniqueIdResetPool ? virtInfo.Index : -1;

src/Avalonia.Layout/NonVirtualizingLayout.cs

@@ -32,7 +32,7 @@ public sealed override void UninitializeForContext(LayoutContext context)
         /// <inheritdoc/>
         public sealed override Size Measure(LayoutContext context, Size availableSize)
         {
-            return MeasureOverride((VirtualizingLayoutContext)context, availableSize);
+            return MeasureOverride((NonVirtualizingLayoutContext)context, availableSize);
         }
 
         /// <inheritdoc/>
@@ -49,7 +49,7 @@ public sealed override Size Arrange(LayoutContext context, Size finalSize)
         /// The context object that facilitates communication between the layout and its host
         /// container.
         /// </param>
-        protected virtual void InitializeForContextCore(VirtualizingLayoutContext context)
+        protected virtual void InitializeForContextCore(LayoutContext context)
         {
         }
 
@@ -61,7 +61,7 @@ protected virtual void InitializeForContextCore(VirtualizingLayoutContext contex
         /// The context object that facilitates communication between the layout and its host
         /// container.
         /// </param>
-        protected virtual void UninitializeForContextCore(VirtualizingLayoutContext context)
+        protected virtual void UninitializeForContextCore(LayoutContext context)
         {
         }
 
@@ -83,7 +83,7 @@ protected virtual void UninitializeForContextCore(VirtualizingLayoutContext cont
         /// of the allocated sizes for child objects or based on other considerations such as a
         /// fixed container size.
         /// </returns>
-        protected abstract Size MeasureOverride(VirtualizingLayoutContext context, Size availableSize);
+        protected abstract Size MeasureOverride(NonVirtualizingLayoutContext context, Size availableSize);
 
         /// <summary>
         /// When implemented in a derived class, provides the behavior for the "Arrange" pass of

src/Avalonia.Layout/NonVirtualizingLayoutContext.cs

@@ -0,0 +1,11 @@
+// This source file is adapted from the WinUI project.
+// (https://github.com/microsoft/microsoft-ui-xaml)
+//
+// Licensed to The Avalonia Project under MIT License, courtesy of The .NET Foundation.
+
+namespace Avalonia.Layout
+{
+    public abstract class NonVirtualizingLayoutContext : LayoutContext
+    {
+    }
+}