Completion popup wrapping for arrow key navigation

StabilityMatrix.Avalonia/Controls/CodeCompletion/CompletionList.cs

@@ -202,11 +202,11 @@ public class CompletionList : TemplatedControl
         {
             case Key.Down:
                 e.Handled = true;
-                _listBox.SelectIndex(_listBox.SelectedIndex + 1);
+                _listBox.SelectNextIndexWithLoop();
                 break;
             case Key.Up:
                 e.Handled = true;
-                _listBox.SelectIndex(_listBox.SelectedIndex - 1);
+                _listBox.SelectPreviousIndexWithLoop();
                 break;
             case Key.PageDown:
                 e.Handled = true;

StabilityMatrix.Avalonia/Controls/CodeCompletion/CompletionListBox.cs

@@ -91,6 +91,28 @@ public class CompletionListBox : ListBox
         SelectedIndex = -1;
     }
 
+    /// <summary>
+    /// Selects the next item. If the last item is already selected, selects the first item.
+    /// </summary>
+    public void SelectNextIndexWithLoop()
+    {
+        if (ItemCount <= 0)
+            return;
+
+        SelectIndex((SelectedIndex + 1) % ItemCount);
+    }
+
+    /// <summary>
+    /// Selects the previous item. If the first item is already selected, selects the last item.
+    /// </summary>
+    public void SelectPreviousIndexWithLoop()
+    {
+        if (ItemCount <= 0)
+            return;
+
+        SelectIndex((SelectedIndex - 1 + ItemCount) % ItemCount);
+    }
+
     /// <summary>
     /// Selects the item with the specified index and scrolls it into view.
     /// </summary>