Make DropDown mouse selection work.

Perspex.Controls/DropDown.cs

@@ -7,11 +7,13 @@
 namespace Perspex.Controls
 {
     using System;
+    using System.Linq;
     using Generators;
     using Perspex.Controls.Primitives;
     using Perspex.Controls.Templates;
     using Perspex.Input;
     using Perspex.Layout;
+    using VisualTree;
 
     public class DropDown : SelectingItemsControl, IContentControl
     {
@@ -93,8 +95,13 @@ namespace Perspex.Controls
         {
             if (!this.IsDropDownOpen)
             {
-                this.IsDropDownOpen = true;
+                if (((IVisual)e.Source).GetVisualAncestors().Last().GetType() != typeof(PopupRoot))
+                {
+                    this.IsDropDownOpen = true;
+                }
             }
+
+            base.OnPointerPressed(e);
         }
 
         protected override void OnTemplateApplied()

Perspex.Controls/Primitives/Popup.cs

@@ -139,7 +139,8 @@ namespace Perspex.Controls.Primitives
         }
 
         /// <summary>
-        /// Gets or sets a value indicating whether the popup should stay open when the popup loses focus.
+        /// Gets or sets a value indicating whether the popup should stay open when the popup is
+        /// pressed or loses focus.
         /// </summary>
         public bool StaysOpen
         {
@@ -316,13 +317,6 @@ namespace Perspex.Controls.Primitives
         {
             if (!this.StaysOpen)
             {
-                var routed = e as RoutedEventArgs;
-
-                if (routed != null)
-                {
-                    routed.Handled = true;
-                }
-
                 this.Close();
             }
         }