As recommended switch to NewKeyDownEvent

src/UI/Windows/BigListBox.cs

@@ -222,7 +222,7 @@ private void ListView_KeyPress(object? sender, Key key)
         if (key == Key.Backspace || char.IsLetterOrDigit(c))
         {
             this.searchBox?.FocusFirst(TabBehavior.TabStop);
-            this.searchBox?.OnProcessKeyDown(key);
+            this.searchBox?.NewKeyDownEvent(key);
             key.Handled = true;
         }
     }