From my point of view the mega menu of this template is not a real mega menu, as it works like any multi-column menu. When you try to make a wide submenu, the menu does not stop at the right border of the template, but you have to resize any submenu width the nearer you get to the right border or it laps over the right border of the wrapper. Normally, as far as I know a mega menu should always have any submenu start at the left side of the template and fills the full width of the template.
In fact, when I looked at the demo before getting the template, I really thought that this can't be the mega menu but the regular drop down and looked for some way to change the style of the demo, which did not work, lol.
This one below is one of the first mega menus I ever saw, yet very basic in styling but one can see how it should work and so ask if some additional styles can be provided to officially update the MyStore "mega menu" to a real MEGA menu?:)
http://www.otto.de/
Thanks!