Default nice-menu-left style sometimes makes it difficult to reach child items

When I create a menu block whose menu style is "left", I find that trying to reach a third- or fourth- level child item can be quite difficult. For example, suppose I have this kind of menu:

- Movies
-- Comedy
--- Independent
--- Romantic
--- Slapstick
-- Drama
-- Sci-Fi

In a left menu, hovering over "Comedy" and trying to select "Romantic" or "Slapstick" can be quite difficult. I believe the reason is that the width of "Comedy" is 12.5 em, but the ul underneath Comedy is -12.65 em to the left (i.e. 12.65 em to the right), thereby creating a tiny little gap between Comedy and its child ul, thereby allowing users to slip into the gap accidentally. Seems like a minor problem, but in fact it can be quite frustrating.

The attached patch fixes this issue, I think, though I'm not sure if the second style change is necessary, since I can't figure out what that CSS selector does.