Hide scrollbars on ScrollViewer when not needed.

Also added an image size slider to test.

Perspex.Controls/Primitives/ScrollBar.cs

@@ -7,6 +7,7 @@
 namespace Perspex.Controls.Primitives
 {
     using System;
+    using System.Reactive.Linq;
 
     public class ScrollBar : TemplatedControl
     {
@@ -27,6 +28,8 @@ namespace Perspex.Controls.Primitives
 
         public ScrollBar()
         {
+            this.Classes.Add(":vertical");
+
             this.GetObservable(OrientationProperty).Subscribe(o =>
             {
                 if (o == Orientation.Horizontal)

Perspex.Controls/Primitives/Track.cs

@@ -119,9 +119,9 @@ namespace Perspex.Controls.Primitives
             {
                 var range = this.Maximum - this.Minimum;
                 var thumbFraction = this.ViewportSize / range;
-                var valueFraction = this.Value / range;
+                var valueFraction = (this.Value - this.Minimum) / range;
 
-                if (double.IsNaN(thumbFraction))
+                if (double.IsNaN(thumbFraction) || double.IsInfinity(thumbFraction))
                 {
                     thumbFraction = 0;
                 }

Perspex.Controls/ScrollViewer.cs

@@ -60,9 +60,9 @@ namespace Perspex.Controls
                 this.GetObservable(ViewportProperty).StartWith(this.Viewport))
                 .Select(x => new { Extent = x[0], Viewport = x[1] });
 
-            //this.horizontalScrollBar.Bind(
-            //    IsVisibleProperty,
-            //    extentAndViewport.Select(x => x.Extent.Width > x.Viewport.Width));
+            this.horizontalScrollBar.Bind(
+                IsVisibleProperty,
+                extentAndViewport.Select(x => x.Extent.Width > x.Viewport.Width));
 
             this.horizontalScrollBar.Bind(
                 ScrollBar.MaximumProperty,
@@ -72,9 +72,9 @@ namespace Perspex.Controls
                 ScrollBar.ViewportSizeProperty,
                 extentAndViewport.Select(x => x.Viewport.Width));
 
-            //this.verticalScrollBar.Bind(
-            //    IsVisibleProperty,
-            //    extentAndViewport.Select(x => x.Extent.Height > x.Viewport.Height));
+            this.verticalScrollBar.Bind(
+                IsVisibleProperty,
+                extentAndViewport.Select(x => x.Extent.Height > x.Viewport.Height));
 
             this.verticalScrollBar.Bind(
                 ScrollBar.MaximumProperty,

Perspex.Layout/LayoutManager.cs

@@ -37,9 +37,9 @@ namespace Perspex.Layout
 
         public void ExecuteLayoutPass()
         {
+            this.LayoutQueued = false;
             this.root.Measure(this.root.ClientSize);
             this.root.Arrange(new Rect(this.root.ClientSize));
-            this.LayoutQueued = false;
         }
 
         public void InvalidateMeasure(ILayoutable item)

TestApplication/Program.cs

@@ -203,6 +203,8 @@ namespace TestApplication
 
         private static TabItem ImagesTab()
         {
+            ScrollBar size;
+
             return new TabItem
             {
                 Header = "Images",
@@ -214,6 +216,13 @@ namespace TestApplication
                     Gap = 8,
                     Children = new Controls
                     {
+                        (size = new ScrollBar
+                        {
+                            Minimum = 100,
+                            Maximum = 400,
+                            Value = 400,
+                            Orientation = Orientation.Horizontal,
+                        }),
                         new ScrollViewer
                         {
                             Width = 200,
@@ -221,10 +230,10 @@ namespace TestApplication
                             Content = new Image
                             {
                                 Source = new Bitmap("github_icon.png"),
-                                Width = 400,
-                                Height = 400,
+                                [!Image.WidthProperty] = size[!ScrollBar.ValueProperty],
+                                [!Image.HeightProperty] = size[!ScrollBar.ValueProperty],
                             },
-                        }
+                        },
                     }
                 },
             };