Added page up/down support to virt lists.

src/Avalonia.Controls/Presenters/ItemVirtualizerSimple.cs

@@ -7,6 +7,7 @@ using System.Collections.Specialized;
 using System.Linq;
 using Avalonia.Controls.Utils;
 using Avalonia.Input;
+using Avalonia.Utilities;
 
 namespace Avalonia.Controls.Presenters
 {
@@ -165,6 +166,12 @@ namespace Avalonia.Controls.Presenters
                     case NavigationDirection.Down:
                         newItemIndex = itemIndex + 1;
                         break;
+                    case NavigationDirection.PageUp:
+                        newItemIndex = Math.Max(0, itemIndex - (int)ViewportValue);
+                        break;
+                    case NavigationDirection.PageDown:
+                        newItemIndex = Math.Min(ItemCount - 1, itemIndex + (int)ViewportValue);
+                        break;
                 }
             }
             else
@@ -177,6 +184,12 @@ namespace Avalonia.Controls.Presenters
                     case NavigationDirection.Right:
                         newItemIndex = itemIndex + 1;
                         break;
+                    case NavigationDirection.PageUp:
+                        newItemIndex = Math.Max(0, itemIndex - (int)ViewportValue);
+                        break;
+                    case NavigationDirection.PageDown:
+                        newItemIndex = Math.Min(ItemCount - 1, itemIndex + (int)ViewportValue);
+                        break;
                 }
             }
 
@@ -189,7 +202,8 @@ namespace Avalonia.Controls.Presenters
 
                 if (newItemIndex < firstIndex || newItemIndex > lastIndex)
                 {
-                    OffsetValue += newItemIndex - itemIndex;
+                    var newOffset = OffsetValue + (newItemIndex - itemIndex);
+                    OffsetValue = CoerceOffset(newOffset);
                     InvalidateScroll();
                 }
 
@@ -398,5 +412,16 @@ namespace Avalonia.Controls.Presenters
             Owner.ItemContainerGenerator.Dematerialize(FirstIndex + index, count);
             NextIndex -= count;
         }
+
+        /// <summary>
+        /// Ensures an offset value is within the value range.
+        /// </summary>
+        /// <param name="value">The value.</param>
+        /// <returns>The coerced value.</returns>
+        private double CoerceOffset(double value)
+        {
+            var max = Math.Max(ExtentValue - ViewportValue, 0);
+            return MathUtilities.Clamp(value, 0, max);
+        }
     }
 }

src/Avalonia.Input/KeyboardNavigationHandler.cs

@@ -117,6 +117,12 @@ namespace Avalonia.Input
                     case Key.Right:
                         direction = NavigationDirection.Right;
                         break;
+                    case Key.PageUp:
+                        direction = NavigationDirection.PageUp;
+                        break;
+                    case Key.PageDown:
+                        direction = NavigationDirection.PageDown;
+                        break;
                 }
 
                 if (direction.HasValue)

src/Avalonia.Input/Navigation/DirectionalNavigation.cs

@@ -35,7 +35,6 @@ namespace Avalonia.Input.Navigation
 
             if (container != null)
             {
-                var isForward = IsForward(direction);
                 var mode = KeyboardNavigation.GetDirectionalNavigation((InputElement)container);
 
                 switch (mode)