Limit initial measure of virtualized list.

Prevents all items being materialized when virtualized control is
measured with infinity.

src/Avalonia.Controls/Presenters/ItemsPresenter.cs

@@ -91,6 +91,26 @@ namespace Avalonia.Controls.Presenters
             _virtualizer?.ScrollIntoView(item);
         }
 
+        protected override Size MeasureOverride(Size availableSize)
+        {
+            // If infinity is passed as the available size and we're virtualized then we need to
+            // fill the available space, but to do that we *don't* want to materialize all our
+            // items! Take a look at the root of the tree for a MaxClientSize and use that as
+            // the available size.
+            if (availableSize == Size.Infinity && VirtualizationMode != ItemVirtualizationMode.None)
+            {
+                var window = VisualRoot as Window;
+
+                if (window != null)
+                {
+                    availableSize = window.PlatformImpl.MaxClientSize;
+                }
+            }
+
+            Panel.Measure(availableSize);
+            return Panel.DesiredSize;
+        }
+
         /// <inheritdoc/>
         protected override void PanelCreated(IPanel panel)
         {

src/Avalonia.Controls/VirtualizingStackPanel.cs

@@ -65,8 +65,6 @@ namespace Avalonia.Controls
         {
             if (availableSize != ((ILayoutable)this).PreviousMeasure)
             {
-                // TODO: We need to put a reasonable limit on this, probably based on the max
-                // window size.
                 _availableSpace = availableSize;
                 Controller?.UpdateControls();
             }