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the sub-menu listing extends below the bottom of the screen.
This is dependent on the size of the user’s monitor. A user with a larger monitor may not have this issue.
That said, if your menus are really long, it can be difficult to navigate, and this is a disincentive for visitors to return. It’s generally best to either reorganize the menu, or to link to a category archive. Menus are most effective when they are short.