Submenu does not move with scrollbar

Question

I have an Infragistics UltraWebMenu control. When the browser's vertical scrollbar is at the top, the submenu items appear nicely.

However, if I do the following steps:

1. Move the vertical scrollbar down.
2. Click one of menu item, so that the submenu comes.
3. Move vertical scrollbar up.

Then the submenu still stay there, not moving along with the vertical scrollbar. This creates a huge vertical gap between main menu and submenu.