Added vertical scrollbar.

STILL VERY HACKY.

Perspex.Controls/Primitives/ScrollBar.cs

@@ -6,6 +6,8 @@
 
 namespace Perspex.Controls.Primitives
 {
+    using System;
+
     public class ScrollBar : TemplatedControl
     {
         public static readonly PerspexProperty<double> MinimumProperty =
@@ -23,6 +25,23 @@ namespace Perspex.Controls.Primitives
         public static readonly PerspexProperty<Orientation> OrientationProperty =
             PerspexProperty.Register<ScrollBar, Orientation>("Orientation");
 
+        public ScrollBar()
+        {
+            this.GetObservable(OrientationProperty).Subscribe(o =>
+            {
+                if (o == Orientation.Horizontal)
+                {
+                    this.Classes.Remove(":vertical");
+                    this.Classes.Add(":horizontal");
+                }
+                else
+                {
+                    this.Classes.Remove(":horizontal");
+                    this.Classes.Add(":vertical");
+                }
+            });
+        }
+
         public double Minimum
         {
             get { return this.GetValue(MinimumProperty); }

Perspex.Controls/Primitives/Track.cs

@@ -51,6 +51,7 @@ namespace Perspex.Controls.Primitives
             AffectsArrange(MinimumProperty);
             AffectsArrange(MaximumProperty);
             AffectsArrange(ValueProperty);
+            AffectsMeasure(OrientationProperty);
         }
 
         public double Minimum
@@ -127,6 +128,9 @@ namespace Perspex.Controls.Primitives
                 }
                 else
                 {
+                    var height = finalSize.Height * thumbsize;
+                    var y = finalSize.Height * (this.Value / range);
+                    thumb.Arrange(new Rect(0, y, finalSize.Width, height));
                 }
             }
 
@@ -136,16 +140,22 @@ namespace Perspex.Controls.Primitives
         private void ThumbDragged(object sender, VectorEventArgs e)
         {
             var range = this.Maximum - this.Minimum;
+            var value = this.Value;
 
             if (this.Orientation == Orientation.Horizontal)
             {
-                var value = this.Value + e.Vector.X * (range / this.ActualSize.Width);
+                value += e.Vector.X * (range / this.ActualSize.Width);
 
-                value = Math.Max(value, this.Minimum);
-                value = Math.Min(value, this.Maximum - this.ViewportSize);
-
-                this.Value = value;
             }
+            else
+            {
+                value += e.Vector.Y * (range / this.ActualSize.Height);
+            }
+
+            value = Math.Max(value, this.Minimum);
+            value = Math.Min(value, this.Maximum - this.ViewportSize);
+
+            this.Value = value;
         }
     }
 }

Perspex.Controls/StackPanel.cs

@@ -114,25 +114,13 @@ namespace Perspex.Controls
 
                 if (Orientation == Orientation.Vertical)
                 {
-                    childWidth = finalSize.Width;
-
                     Rect childFinal = new Rect(0, arrangedHeight, childWidth, childHeight);
-
-                    if (childFinal.IsEmpty)
-                    {
-                        child.Arrange(new Rect());
-                    }
-                    else
-                    {
-                        child.Arrange(childFinal);
-                    }
-
+                    child.Arrange(childFinal);
                     arrangedWidth = Math.Max(arrangedWidth, childWidth);
                     arrangedHeight += childHeight + gap;
                 }
                 else
                 {
-                    childHeight = finalSize.Height;
                     Rect childFinal = new Rect(arrangedWidth, 0, childWidth, childHeight);
                     child.Arrange(childFinal);
                     arrangedWidth += childWidth + gap;

Perspex.Themes.Default/ScrollBarStyle.cs

@@ -24,7 +24,22 @@ namespace Perspex.Themes.Default
                     Setters = new[]
                     {
                         new Setter(ScrollBar.TemplateProperty, ControlTemplate.Create<ScrollBar>(this.Template)),
-                        new Setter(ScrollBar.HeightProperty, 20.0),
+                    },
+                },
+                new Style(x => x.OfType<ScrollBar>().Class(":horizontal"))
+                {
+                    Setters = new[]
+                    {
+                        new Setter(ScrollBar.TemplateProperty, ControlTemplate.Create<ScrollBar>(this.Template)),
+                        new Setter(ScrollBar.HeightProperty, 10.0),
+                    },
+                },
+                new Style(x => x.OfType<ScrollBar>().Class(":vertical"))
+                {
+                    Setters = new[]
+                    {
+                        new Setter(ScrollBar.TemplateProperty, ControlTemplate.Create<ScrollBar>(this.Template)),
+                        new Setter(ScrollBar.WidthProperty, 10.0),
                     },
                 },
             });

TestApplication/Program.cs

@@ -84,8 +84,8 @@ namespace TestApplication
 
         static void Main(string[] args)
         {
-            //LogManager.Enable(new TestLogger());
-            //LogManager.Instance.LogLayoutMessages = true;
+            LogManager.Enable(new TestLogger());
+            LogManager.Instance.LogLayoutMessages = true;
 
             App application = new App
             {
@@ -258,6 +258,13 @@ namespace TestApplication
                     Gap = 8,
                     Children = new Controls
                     {
+                        new ScrollBar
+                        {
+                            Orientation = Orientation.Vertical,
+                            ViewportSize = 25,
+                            Value = 25,
+                            Height = 300,
+                        },
                         new ScrollBar
                         {
                             Orientation = Orientation.Horizontal,