Allow random item heights in VirtualizationDemo.

Add a "randomize" and "reset" button in order to test #2144.

samples/VirtualizationDemo/MainWindow.xaml

@@ -39,6 +39,8 @@
             <Button Command="{Binding RecreateCommand}">Recreate</Button>
             <Button Command="{Binding SelectFirstCommand}">Select First</Button>
             <Button Command="{Binding SelectLastCommand}">Select Last</Button>
+            <Button Command="{Binding RandomizeSize}">Randomize Size</Button>
+            <Button Command="{Binding ResetSize}">Reset Size</Button>
         </StackPanel>
 
         <ListBox Name="listBox" 
@@ -55,7 +57,7 @@
             </ListBox.ItemsPanel>
             <ListBox.ItemTemplate>
                 <DataTemplate>
-                    <TextBlock Text="{Binding Header}" TextWrapping="Wrap"/>
+                    <TextBlock Text="{Binding Header}" Height="{Binding Height}" TextWrapping="Wrap"/>
                 </DataTemplate>
             </ListBox.ItemTemplate>
         </ListBox>

samples/VirtualizationDemo/ViewModels/ItemViewModel.cs

@@ -10,6 +10,7 @@ namespace VirtualizationDemo.ViewModels
     {
         private string _prefix;
         private int _index;
+        private double _height = double.NaN;
 
         public ItemViewModel(int index, string prefix = "Item")
         {
@@ -18,5 +19,11 @@ namespace VirtualizationDemo.ViewModels
         }
 
         public string Header => $"{_prefix} {_index}";
+
+        public double Height
+        {
+            get => _height;
+            set => this.RaiseAndSetIfChanged(ref _height, value);
+        }
     }
 }

samples/VirtualizationDemo/ViewModels/MainWindowViewModel.cs

@@ -98,6 +98,24 @@ namespace VirtualizationDemo.ViewModels
         public ReactiveCommand SelectFirstCommand { get; private set; }
         public ReactiveCommand SelectLastCommand { get; private set; }
 
+        public void RandomizeSize()
+        {
+            var random = new Random();
+
+            foreach (var i in Items)
+            {
+                i.Height = random.Next(240) + 10;
+            }
+        }
+
+        public void ResetSize()
+        {
+            foreach (var i in Items)
+            {
+                i.Height = double.NaN;
+            }
+        }
+
         private void ResizeItems(int count)
         {
             if (Items == null)