Added initial changes to allow the user to change the selected item while the dropdown is closed but selected and allows the user to open the dropdown by pressing down

src/Avalonia.Controls/DropDown.cs

@@ -114,6 +114,30 @@ namespace Avalonia.Controls
                     IsDropDownOpen = false;
                     e.Handled = true;
                 }
+
+                if (!IsDropDownOpen)
+                {
+                    if (e.Key == Key.Right)
+                    {
+                        if (++SelectedIndex >= ItemCount)
+                            SelectedIndex = 0;
+                        
+                        e.Handled = true;
+                    }
+                    else if (e.Key == Key.Left)
+                    {
+                        if (--SelectedIndex < 0)
+                            SelectedIndex = ItemCount - 1;
+                        e.Handled = true;
+                    }
+                    else if (e.Key == Key.Down)
+                    {
+                        IsDropDownOpen = true;
+                        if (SelectedIndex == -1)
+                            SelectedIndex = 0;
+                        e.Handled = true;
+                    }
+                }
             }
         }
 

src/Avalonia.Controls/ItemsControl.cs

@@ -11,6 +11,7 @@ using Avalonia.Controls.Presenters;
 using Avalonia.Controls.Primitives;
 using Avalonia.Controls.Templates;
 using Avalonia.Controls.Utils;
+using Avalonia.Input;
 using Avalonia.LogicalTree;
 using Avalonia.Metadata;
 
@@ -106,6 +107,12 @@ namespace Avalonia.Controls
             set { SetAndRaise(ItemsProperty, ref _items, value); }
         }
 
+        public int ItemCount
+        {
+            get;
+            private set;
+        }
+
         /// <summary>
         /// Gets or sets the panel used to display the items.
         /// </summary>
@@ -352,6 +359,10 @@ namespace Avalonia.Controls
                     RemoveControlItemsFromLogicalChildren(e.OldItems);
                     break;
             }
+            
+            int? count = (Items as IList)?.Count;
+            if (count != null)
+                ItemCount = (int)count;
 
             var collection = sender as ICollection;
             PseudoClasses.Set(":empty", collection == null || collection.Count == 0);