Added arrow key scroll navigation to ScrollViewer control (#20482)

* Added arrow key scroll navigation to ScrollViewer control

* Check event source before handling Left/Right/Up/Down keys

* Handle right-to-left flow direction in ScrollViewer keys

* Added ScrollViewer test

* Added ScrollBar test

---------

Co-authored-by: Julien Lebosquain <julien@lebosquain.net>

src/Avalonia.Controls/Primitives/ScrollBar.cs

@@ -250,15 +250,41 @@ namespace Avalonia.Controls.Primitives
 
         protected override void OnKeyDown(KeyEventArgs e)
         {
-            if (e.Key == Key.PageUp)
+            if (Orientation == Orientation.Vertical)
             {
-                LargeDecrement();
-                e.Handled = true;
+                if (e.Key == Key.PageUp)
+                {
+                    LargeDecrement();
+                    e.Handled = true;
+                }
+                else if (e.Key == Key.PageDown)
+                {
+                    LargeIncrement();
+                    e.Handled = true;
+                }
+                else if (e.Key == Key.Up)
+                {
+                    SmallDecrement();
+                    e.Handled = true;
+                }
+                else if (e.Key == Key.Down)
+                {
+                    SmallIncrement();
+                    e.Handled = true;
+                }
             }
-            else if (e.Key == Key.PageDown)
+            else if (Orientation == Orientation.Horizontal)
             {
-                LargeIncrement();
-                e.Handled = true;
+                if (e.Key == Key.Left)
+                {
+                    SmallDecrement();
+                    e.Handled = true;
+                }
+                else if (e.Key == Key.Right)
+                {
+                    SmallIncrement();
+                    e.Handled = true;
+                }
             }
         }
 

src/Avalonia.Controls/ScrollViewer.cs

@@ -806,6 +806,36 @@ namespace Avalonia.Controls
                 PageDown();
                 e.Handled = true;
             }
+            else if (e.Source == this)
+            {
+                bool rtl = FlowDirection == Media.FlowDirection.RightToLeft;
+                if (e.Key == Key.Left)
+                {
+                    if (rtl)
+                        LineRight();
+                    else
+                        LineLeft();
+                    e.Handled = true;
+                }
+                else if (e.Key == Key.Right)
+                {
+                    if (rtl)
+                        LineLeft();
+                    else
+                        LineRight();
+                    e.Handled = true;
+                }
+                else if (e.Key == Key.Up)
+                {
+                    LineUp();
+                    e.Handled = true;
+                }
+                else if (e.Key == Key.Down)
+                {
+                    LineDown();
+                    e.Handled = true;
+                }
+            }
         }
 
         /// <summary>

tests/Avalonia.Controls.UnitTests/Primitives/ScrollBarTests.cs

@@ -379,6 +379,55 @@ namespace Avalonia.Controls.UnitTests.Primitives
             Assert.False(args.Handled);
         }
 
+        [Fact]
+        public void Focus_KeyInput_Should_Scroll()
+        {
+            // Vertical scrollbar
+            var target = CreateScrollBar(Orientation.Vertical);
+
+            // Page down and page up
+            KeyDown(target, Key.PageDown);
+            Assert.Equal(target.LargeChange, target.Value);
+            KeyDown(target, Key.PageUp);
+            Assert.Equal(0, target.Value);
+
+            // Small scrolling with arrow keys
+            KeyDown(target, Key.Down);
+            Assert.Equal(target.SmallChange, target.Value);
+            KeyDown(target, Key.Up);
+            Assert.Equal(0, target.Value);
+
+            // Vertical scrollbars shouldn't scroll for left and right keys
+            KeyDown(target, Key.Right);
+            Assert.Equal(0, target.Value);
+            KeyDown(target, Key.Left);
+            Assert.Equal(0, target.Value);
+
+            // Horizontal scrollbar
+            target = CreateScrollBar(Orientation.Horizontal);
+
+            // Small scrolling with arrow keys
+            KeyDown(target, Key.Right);
+            Assert.Equal(target.SmallChange, target.Value);
+            KeyDown(target, Key.Left);
+            Assert.Equal(0, target.Value);
+
+            // Horizontal scrollbars shouldn't scroll for up and down keys
+            KeyDown(target, Key.Down);
+            Assert.Equal(0, target.Value);
+            KeyDown(target, Key.Up);
+            Assert.Equal(0, target.Value);
+        }
+
+        private static void KeyDown(IInputElement target, Key key)
+        {
+            target.RaiseEvent(new KeyEventArgs
+            {
+                RoutedEvent = InputElement.KeyDownEvent,
+                Key = key,
+            });
+        }
+
         private static ContextRequestedEventArgs CreateContextRequested(ScrollBar target, PointerType pointerType)
         {
             var pointer = new Pointer(Pointer.GetNextFreeId(), pointerType, true);

tests/Avalonia.Controls.UnitTests/ScrollViewerTests.cs

@@ -4,6 +4,7 @@ using System.Linq;
 using Avalonia.Controls.Presenters;
 using Avalonia.Controls.Primitives;
 using Avalonia.Controls.Templates;
+using Avalonia.Input;
 using Avalonia.Layout;
 using Avalonia.Media;
 using Avalonia.UnitTests;
@@ -583,6 +584,55 @@ namespace Avalonia.Controls.UnitTests
             Assert.Equal(1, panel.ArrangeOverrideCalls);
         }
 
+        [Fact]
+        public void Focus_KeyInput_Should_Scroll()
+        {
+            var panel = new Panel
+            {
+                Width = 100_000,
+                Height = 100_000,
+            };
+            var target = new ScrollViewer
+            {
+                Content = panel,
+                Template = new FuncControlTemplate<ScrollViewer>(CreateTemplate),
+            };
+            var root = new TestRoot(target);
+            root.LayoutManager.ExecuteInitialLayoutPass();
+
+            // Page down and page up
+            KeyDown(target, Key.PageDown);
+            Assert.Equal(new(0, target.Viewport.Height), target.Offset);
+            KeyDown(target, Key.PageUp);
+            Assert.Equal(new(0, 0), target.Offset);
+
+            // Per-line scrolling in all directions with arrow keys
+            KeyDown(target, Key.Down);
+            Assert.Equal(new(0, target.SmallChange.Height), target.Offset);
+            KeyDown(target, Key.Right);
+            Assert.Equal(new(target.SmallChange.Width, target.SmallChange.Height), target.Offset);
+            KeyDown(target, Key.Up);
+            Assert.Equal(new(ScrollViewer.DefaultSmallChange, 0), target.Offset);
+            KeyDown(target, Key.Left);
+            Assert.Equal(new(0, 0), target.Offset);
+
+            // Scrolling horizontally with a right-to-left flow direction
+            target.FlowDirection = FlowDirection.RightToLeft;
+            KeyDown(target, Key.Left);
+            Assert.Equal(new(target.SmallChange.Width, 0), target.Offset);
+            KeyDown(target, Key.Right);
+            Assert.Equal(new(0, 0), target.Offset);
+        }
+
+        private static void KeyDown(IInputElement target, Key key)
+        {
+            target.RaiseEvent(new KeyEventArgs
+            {
+                RoutedEvent = InputElement.KeyDownEvent,
+                Key = key,
+            });
+        }
+
         public class TestPanel : Panel
         {
             public int DesiredWidth { get; set; }