Added Popup.PlacementMode.

Hacky: only supports Bottom and Right so far, and doesn't take into
account screen edges etc.

Perspex.Controls/Perspex.Controls.csproj

@@ -50,6 +50,7 @@
     <Compile Include="Deck.cs" />
     <Compile Include="Platform\IPopupImpl.cs" />
     <Compile Include="Platform\ITopLevelImpl.cs" />
+    <Compile Include="PlacementMode.cs" />
     <Compile Include="Popup.cs" />
     <Compile Include="IDataTemplate.cs" />
     <Compile Include="Presenters\IItemsPresenter.cs" />

Perspex.Controls/PlacementMode.cs

@@ -0,0 +1,14 @@
+// -----------------------------------------------------------------------
+// <copyright file="PlacementMode.cs" company="Steven Kirk">
+// Copyright 2015 MIT Licence. See licence.md for more information.
+// </copyright>
+// -----------------------------------------------------------------------
+
+namespace Perspex.Controls
+{
+    public enum PlacementMode
+    {
+        Bottom,
+        Right,
+    }
+}

Perspex.Controls/Popup.cs

@@ -10,6 +10,7 @@ namespace Perspex.Controls
     using Perspex.Interactivity;
     using Perspex.Platform;
     using Perspex.Rendering;
+    using Perspex.VisualTree;
 
     public class Popup : Control
     {
@@ -19,6 +20,9 @@ namespace Perspex.Controls
         public static readonly PerspexProperty<bool> IsOpenProperty =
             PerspexProperty.Register<Popup, bool>("IsOpen");
 
+        public static readonly PerspexProperty<PlacementMode> PlacementModeProperty =
+            PerspexProperty.Register<Popup, PlacementMode>("PlacementMode", defaultValue: PlacementMode.Bottom);
+
         public static readonly PerspexProperty<Control> PlacementTargetProperty =
             PerspexProperty.Register<Popup, Control>("PlacementTarget");
 
@@ -61,6 +65,12 @@ namespace Perspex.Controls
             set { this.SetValue(IsOpenProperty, value); }
         }
 
+        public PlacementMode PlacementMode
+        {
+            get { return this.GetValue(PlacementModeProperty); }
+            set { this.SetValue(PlacementModeProperty, value); }
+        }
+
         public Control PlacementTarget
         {
             get { return this.GetValue(PlacementTargetProperty); }
@@ -130,9 +140,24 @@ namespace Perspex.Controls
 
         private Point GetPosition()
         {
-            if (this.PlacementTarget != null)
+            var target = this.PlacementTarget ?? this.GetVisualParent<Control>();
+            Point point;
+
+            switch (this.PlacementMode)
+            {
+                case PlacementMode.Bottom:
+                    point = target.Bounds.BottomLeft;
+                    break;
+                case PlacementMode.Right:
+                    point = target.Bounds.TopRight;
+                    break;
+                default:
+                    throw new InvalidOperationException("Invalid value for Popup.PlacementMode");
+            }
+
+            if (target != null)
             {
-                return this.PlacementTarget.PointToScreen(new Point(0, this.PlacementTarget.Bounds.Size.Height));
+                return target.PointToScreen(point);
             }
             else
             {

Perspex.Themes.Default/MenuItemStyle.cs

@@ -188,6 +188,7 @@ namespace Perspex.Themes.Default
                         (popup = new Popup
                         {
                             Name = "popup",
+                            PlacementMode = PlacementMode.Right,
                             StaysOpen = true,
                             [!!Popup.IsOpenProperty] = control[!!MenuItem.IsSubMenuOpenProperty],
                             Child = new Border