Right click context menu

Can anybody explain to me how the current right click context menu is built id dtree?
This is very handy in the admin panel.

Not that I want to modify the admin panel but I want to build a custom one for a site.

Hope that someone can give me good pointers. I am having trouble getting the context menu in the right place. (at my mouse pointer)
ALso wonder How you managed to het menux to work....

THanks a lot.