Mega Menu in Adaptive theme feeling like mobile

The proposed solution above may be the best way to go for custom work with your particular use case, or when you have trouble with the stock themes.

Do not discourage yourself by saying "never" for the solution offered, otherwise hire someone else who can to do it for you. If you can't, please be bold to ask uncle Google for "CSS media query". Select the words in the quotes on the left, right click, and hit "Search Google ....", or put them in the browser address bar, and hit Enter.

You have been very courageous to ask here, so you should be to ask uncle Google, too :p

To have better chance for help, theme related questions may need more visual or actual hintings:
A screenshot, or a link to website will do.

If you can't provide such info, unless you can provide better wordings, I don't think your current info is clear enough to help.

Most likely you need this to prevent absolute position from collapsing, assumed without media query:

/* change .menu to your relevant trouble selector */
.menu {
  width: 100%; /* Or 960px, etc */
}

You may also want to remove position: relative; of the direct parent LI element, or easier to override it with position: static;, and assign relative position to the closest UL or NAVBAR element instead. This way you can have the magamenu flyout 100% width spanning across menu items, otherwise the flyout will be shrinked to the nearest narrow LI element.

But that is a wild guess. I recommend using Firebug for FF, or hit f12 for chrome to help you any better yourself.