Set width of a "Position: fixed" div relative to parent div

I'm trying to give a div (position: fixed) the width of 100% (relating to it's parent div). But I've got some problems...

EDIT: The first problem is sovled by using inherit, but it still doesn't work. I think the problem is that I'm using multiple divs that take the 100%/inherit width. You can find the second problem on the jsfiddle update: http://jsfiddle.net/4bGqF/7/

Fox example

#container {
    width: 800px;
}

#fixed {
    position: fixed;
    width: 100%;
}

and the html

<div id="container">
    <div id="fixed">Sitename</div>
    <p>
        blaat
    </p>
</div>

Or you can try it out: http://jsfiddle.net/4bGqF/

The problems seems to be that the fixed element is always taking the width of the window/document. Does anyone know how the fix this?

I can't give my fixed element a fixed with, because I'm using the jScrollPane plugin. It depends on the content whether there's a scrollbar or not.

Thanks a lot!

PS: The text of the 2 divs are on top of each other. This is just an example so that doesn't really matter.

Solution 1:

I´m not sure as to what the second problem is (based on your edit), but if you apply width:inherit to all inner divs, it works: http://jsfiddle.net/4bGqF/9/

You might want to look into a javascript solution for browsers that you need to support and that don´t support width:inherit

Solution 2:

As many people have commented, responsive design very often sets width by %

width:inherit will inherit the CSS width NOT the computed width -- Which means the child container inherits width:100%

But, I think, almost as often responsive design sets max-width too, therefore:

#container {
    width:100%;
    max-width:800px;
}
#contained {
    position:fixed;
    width:inherit;
    max-width:inherit;
}

This worked very satisfyingly to solve my problem of making a sticky menu be restrained to the original parent width whenever it got "stuck"

Both the parent and child will adhere to the width:100% if the viewport is less than the maximum width. Likewise, both will adhere to the max-width:800px when the viewport is wider.

It works with my already responsive theme in a way that I can alter the parent container without having to also alter the fixed child element -- elegant and flexible

ps: I personally think it does not matter one bit that IE6/7 do not use inherit