Test scrollbar auto-hiding.

Perspex.Controls/Primitives/ScrollBar.cs

@@ -9,7 +9,6 @@ namespace Perspex.Controls.Primitives
     using System;
     using System.Reactive;
     using System.Reactive.Linq;
-    using Perspex.Controls.Templates;
 
     /// <summary>
     /// A scrollbar control.
@@ -98,7 +97,7 @@ 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.");

Tests/Perspex.Controls.UnitTests/Primitives/ScrollBarTests.cs

@@ -61,6 +61,58 @@ namespace Perspex.Controls.UnitTests.Primitives
             Assert.Equal(target.Value, 50);
         }
 
+        [Fact]
+        public void ScrollBar_Can_AutoHide()
+        {
+            var target = new ScrollBar();
+
+            target.Visibility = ScrollBarVisibility.Auto;
+            target.Minimum = 0;
+            target.Maximum = 100;
+            target.ViewportSize = 100;
+
+            Assert.False(target.IsVisible);
+        }
+
+        [Fact]
+        public void ScrollBar_Should_Not_AutoHide_When_ViewportSize_Is_NaN()
+        {
+            var target = new ScrollBar();
+
+            target.Visibility = ScrollBarVisibility.Auto;
+            target.Minimum = 0;
+            target.Maximum = 100;
+            target.ViewportSize = double.NaN;
+
+            Assert.True(target.IsVisible);
+        }
+
+        [Fact]
+        public void ScrollBar_Should_Not_AutoHide_When_Visibility_Set_To_Visible()
+        {
+            var target = new ScrollBar();
+
+            target.Visibility = ScrollBarVisibility.Visible;
+            target.Minimum = 0;
+            target.Maximum = 100;
+            target.ViewportSize = 100;
+
+            Assert.True(target.IsVisible);
+        }
+
+        [Fact]
+        public void ScrollBar_Should_Hide_When_Visibility_Set_To_Hidden()
+        {
+            var target = new ScrollBar();
+
+            target.Visibility = ScrollBarVisibility.Hidden;
+            target.Minimum = 0;
+            target.Maximum = 100;
+            target.ViewportSize = 10;
+
+            Assert.False(target.IsVisible);
+        }
+
         private static Control Template(ScrollBar control)
         {
             return new Border