Fix scrollbar values.

Perspex.Controls/Primitives/Track.cs

@@ -118,18 +118,24 @@ namespace Perspex.Controls.Primitives
             if (thumb != null)
             {
                 var range = this.Maximum - this.Minimum;
-                var thumbsize = this.ViewportSize / range;
+                var thumbFraction = this.ViewportSize / range;
+                var valueFraction = this.Value / range;
+
+                if (double.IsNaN(thumbFraction))
+                {
+                    thumbFraction = 0;
+                }
 
                 if (this.Orientation == Orientation.Horizontal)
                 {
-                    var width = finalSize.Width * thumbsize;
-                    var x = finalSize.Width * (this.Value / range);
+                    var width = finalSize.Width * thumbFraction;
+                    var x = (finalSize.Width - thumb.ActualSize.Width) * valueFraction;
                     thumb.Arrange(new Rect(x, 0, width, finalSize.Height));
                 }
                 else
                 {
-                    var height = finalSize.Height * thumbsize;
-                    var y = finalSize.Height * (this.Value / range);
+                    var height = finalSize.Height * thumbFraction;
+                    var y = (finalSize.Height - thumb.ActualSize.Height) * valueFraction;
                     thumb.Arrange(new Rect(0, y, finalSize.Width, height));
                 }
             }
@@ -139,23 +145,26 @@ namespace Perspex.Controls.Primitives
 
         private void ThumbDragged(object sender, VectorEventArgs e)
         {
-            var range = this.Maximum - this.Minimum;
-            var value = this.Value;
+            double range = this.Maximum - this.Minimum;
+            double value = this.Value;
+            double offset;
 
             if (this.Orientation == Orientation.Horizontal)
             {
-                value += e.Vector.X * (range / this.ActualSize.Width);
-
+                offset = e.Vector.X / ((this.ActualSize.Width - this.Thumb.ActualSize.Width) / range);
             }
             else
             {
-                value += e.Vector.Y * (range / this.ActualSize.Height);
+                offset = e.Vector.Y * (range / (this.ActualSize.Height - this.Thumb.ActualSize.Height));
             }
 
-            value = Math.Max(value, this.Minimum);
-            value = Math.Min(value, this.Maximum - this.ViewportSize);
-
-            this.Value = value;
+            if (!double.IsNaN(offset) && !double.IsInfinity(offset))
+            {
+                value += offset;
+                value = Math.Max(value, this.Minimum);
+                value = Math.Min(value, this.Maximum);
+                this.Value = value;
+            }
         }
     }
 }

Perspex.Layout/Layoutable.cs

@@ -125,6 +125,11 @@ namespace Perspex.Layout
 
         public void Measure(Size availableSize)
         {
+            if (double.IsNaN(availableSize.Width) || double.IsNaN(availableSize.Height))
+            {
+                throw new InvalidOperationException("Cannot call Measure using a size with NaN values.");
+            }
+
             availableSize = availableSize.Deflate(this.Margin);
             this.DesiredSize = this.MeasureCore(availableSize).Constrain(availableSize);
 
@@ -137,6 +142,13 @@ namespace Perspex.Layout
 
         public void Arrange(Rect rect)
         {
+            if (rect.Width < 0 || rect.Height < 0 ||
+                double.IsInfinity(rect.Width) || double.IsInfinity(rect.Height) ||
+                double.IsNaN(rect.Width) || double.IsNaN(rect.Height))
+            {
+                throw new InvalidOperationException("Invalid Arrange rectangle.");
+            }
+
             this.Log().Debug(
                 "Arrange of {0} (#{1:x8}) gave {2} ",
                 this.GetType().Name,

Perspex.Themes.Default/ScrollBarStyle.cs

@@ -34,6 +34,13 @@ namespace Perspex.Themes.Default
                         new Setter(ScrollBar.HeightProperty, 10.0),
                     },
                 },
+                new Style(x => x.OfType<ScrollBar>().Class(":horizontal").Template().Id("thumb"))
+                {
+                    Setters = new[]
+                    {
+                        new Setter(Thumb.MinWidthProperty, 10.0),
+                    },
+                },
                 new Style(x => x.OfType<ScrollBar>().Class(":vertical"))
                 {
                     Setters = new[]
@@ -42,6 +49,13 @@ namespace Perspex.Themes.Default
                         new Setter(ScrollBar.WidthProperty, 10.0),
                     },
                 },
+                new Style(x => x.OfType<ScrollBar>().Class(":vertical").Template().Id("thumb"))
+                {
+                    Setters = new[]
+                    {
+                        new Setter(Thumb.MinHeightProperty, 10.0),
+                    },
+                },
             });
         }
 
@@ -59,6 +73,7 @@ namespace Perspex.Themes.Default
                     [~Track.OrientationProperty] = control[~ScrollBar.OrientationProperty],
                     Thumb = new Thumb
                     {
+                        Id = "thumb",
                         Template = ControlTemplate.Create<Thumb>(this.ThumbTemplate),
                     },
                 },

TestApplication/Program.cs

@@ -273,7 +273,6 @@ namespace TestApplication
                         new ScrollBar
                         {
                             Orientation = Orientation.Vertical,
-                            ViewportSize = 25,
                             Value = 25,
                             Height = 300,
                             [Grid.ColumnProperty] = 0,