Make DropDown's dropdown a ListBox.

Perspex.Controls/Primitives/ScrollBar.cs

@@ -100,7 +100,7 @@ namespace Perspex.Controls.Primitives
 
                 case ScrollBarVisibility.Auto:
                     var viewportSize = this.ViewportSize;
-                    return double.IsNaN(viewportSize) || viewportSize < this.Maximum - this.Minimum;
+                    return !double.IsNaN(viewportSize) && viewportSize < this.Maximum - this.Minimum;
 
                 default:
                     throw new InvalidOperationException("Invalid value for ScrollBar.Visibility.");

Perspex.Controls/Primitives/ScrollBarVisibility.cs

@@ -8,8 +8,8 @@ namespace Perspex.Controls.Primitives
 {
     public enum ScrollBarVisibility
     {
+        Auto,
         Visible,
         Hidden,
-        Auto,
     }
 }

Perspex.Controls/ScrollViewer.cs

@@ -65,20 +65,20 @@ namespace Perspex.Controls
                 .Select(x => new { Extent = x[0], Viewport = x[1] });
 
             this.Bind(
-                HorizontalScrollBarMaximumProperty,
-                extentAndViewport.Select(x => Math.Max(x.Extent.Width - x.Viewport.Width, 0)));
+                VerticalScrollBarViewportSizeProperty,
+                extentAndViewport.Select(x => Max((x.Viewport.Height / x.Extent.Height) * (x.Extent.Height - x.Viewport.Height), 0)));
 
             this.Bind(
                 HorizontalScrollBarViewportSizeProperty,
-                extentAndViewport.Select(x => Math.Max((x.Viewport.Width / x.Extent.Width) * (x.Extent.Width - x.Viewport.Width), 0)));
+                extentAndViewport.Select(x => Max((x.Viewport.Width / x.Extent.Width) * (x.Extent.Width - x.Viewport.Width), 0)));
 
             this.Bind(
-                VerticalScrollBarMaximumProperty,
-                extentAndViewport.Select(x => Math.Max(x.Extent.Height - x.Viewport.Height, 0)));
+                HorizontalScrollBarMaximumProperty,
+                extentAndViewport.Select(x => Max(x.Extent.Width - x.Viewport.Width, 0)));
 
             this.Bind(
-                VerticalScrollBarViewportSizeProperty,
-                extentAndViewport.Select(x => Math.Max((x.Viewport.Height / x.Extent.Height) * (x.Extent.Height - x.Viewport.Height), 0)));
+                VerticalScrollBarMaximumProperty,
+                extentAndViewport.Select(x => Max(x.Extent.Height - x.Viewport.Height, 0)));
 
             this.GetObservable(OffsetProperty).Subscribe(x =>
             {
@@ -139,6 +139,12 @@ namespace Perspex.Controls
             return (value < min) ? min : (value > max) ? max : value;
         }
 
+        private static double Max(double x, double y)
+        {
+            var result = Math.Max(x, y);
+            return double.IsNaN(result) ? 0 : Math.Round(result);
+        }
+
         private static Vector CoerceOffset(PerspexObject o, Vector value)
         {
             ScrollViewer scrollViewer = o as ScrollViewer;

Perspex.Layout/Layoutable.cs

@@ -221,6 +221,14 @@ namespace Perspex.Layout
         {
             var parent = this.GetVisualParent<ILayoutable>();
 
+            if (this.IsMeasureValid)
+            {
+                this.Log().Debug(
+                    "Invalidated measure of {0} (#{1:x8})",
+                    this.GetType().Name,
+                    this.GetHashCode());
+            }
+
             this.IsMeasureValid = false;
             this.IsArrangeValid = false;
             this.previousMeasure = null;
@@ -245,6 +253,14 @@ namespace Perspex.Layout
         {
             var root = this.GetLayoutRoot();
 
+            if (this.IsArrangeValid)
+            {
+                this.Log().Debug(
+                    "Invalidated arrange of {0} (#{1:x8})",
+                    this.GetType().Name,
+                    this.GetHashCode());
+            }
+
             this.IsArrangeValid = false;
             this.previousArrange = null;
 

Perspex.Themes.Default/DropDownStyle.cs

@@ -81,7 +81,7 @@ namespace Perspex.Themes.Default
                         },
                         new Popup
                         {
-                            Child = new ItemsControl
+                            Child = new ListBox
                             {
                                 [~ListBox.ItemsProperty] = control[~DropDown.ItemsProperty],
                             },