Centered content with header from left to center

[lkkl2004]

I didn't know what would be the best title for my question...Well, I'll show what I want to do with a simple picture:

The issue:

• I have my content centered, lets say it's 980px width.
• Now, my header will have a logo and a menu.
• BUT, I want my header touse only a left side of the top, like 50% of the content width andthen stretch to the left side of the viewport. I know this ispossible using javascript and calculate the left offset of thecontent and set the dynamic width div with it's left offset.

Is it the best way? I believe there should be another way, but I can't think of one. Hope you guys have a better idea than mine, since mine slows down the perfomance of the site when I resize the window.