Set dropdown popup min width.

Perspex.Controls/Popup.cs

@@ -76,9 +76,17 @@ namespace Perspex.Controls
         {
             if (this.popupRoot == null)
             {
-                this.popupRoot = new PopupRoot();
-                this.popupRoot.Parent = this;
-                this.popupRoot[~PopupRoot.ContentProperty] = this[~ChildProperty];
+                this.popupRoot = new PopupRoot()
+                {
+                    Parent = this,
+                    [~PopupRoot.ContentProperty] = this[~ChildProperty],
+                    [~PopupRoot.WidthProperty] = this[~WidthProperty],
+                    [~PopupRoot.HeightProperty] = this[~HeightProperty],
+                    [~PopupRoot.MinWidthProperty] = this[~MinWidthProperty],
+                    [~PopupRoot.MaxWidthProperty] = this[~MaxWidthProperty],
+                    [~PopupRoot.MinHeightProperty] = this[~MinHeightProperty],
+                    [~PopupRoot.MaxHeightProperty] = this[~MaxHeightProperty],
+                };
             }
 
             this.popupRoot.SetPosition(this.GetPosition());
@@ -101,6 +109,11 @@ namespace Perspex.Controls
             this.IsOpen = false;
         }
 
+        protected override Size MeasureCore(Size availableSize)
+        {
+            return new Size();
+        }
+
         protected override void OnAttachedToVisualTree(IRenderRoot root)
         {
             base.OnAttachedToVisualTree(root);

Perspex.Themes.Default/DropDownStyle.cs

@@ -7,6 +7,7 @@
 namespace Perspex.Themes.Default
 {
     using System.Linq;
+    using System.Reactive.Linq;
     using Perspex.Controls;
     using Perspex.Controls.Presenters;
     using Perspex.Controls.Primitives;
@@ -89,6 +90,7 @@ namespace Perspex.Themes.Default
                             PlacementTarget = control,
                             StaysOpen = false,
                             [~~Popup.IsOpenProperty] = control[~~DropDown.IsDropDownOpenProperty],
+                            [~Popup.MinWidthProperty] = control[~DropDown.ActualSizeProperty].Cast<Size>().Select(x => (object)x.Width),
                         }
                     },
                 },