position:fixed and width:inherit with percentage parent

Question

I\'m trying to give a fixed element a width of a percentage parent (here #container). When I use pixel instead of the percentage then it works

Answer 1

set the width of "fixed" to 100%, and give it (let's say) a position: relative instead of fixed. http://jsfiddle.net/J7yE4/

#fixed {
    position: relative;
    width: 100%;
    border: 1px solid green;
}

Answer 2

It seems as though the static value (250px) can be propagated through normal inheritance. Whereas when the relative value (90%) is being used, the fixed div has already been taken out-of-flow, and now inheritance-wise, it's parent is the viewport. It seems to me that you're going to have to use good old js.

But, this question is months old now, so it probably doesn't matter either way.

Answer 3

There's a belief that inherited values are 'translated' from relative ones (such as percentages) into absolute ones. Guess what? It's wrong. Here's what MDN says about it:

The inherit CSS-value causes the element for which it is specified to take the computed value of the property from its parent element.

[...]

The computation needed to reach the computed value for the property typically involves converting relative values (such as those in em units or percentages) to absolute values. For example, if an element has specified values font-size: 16px and padding-top: 2em, then the computed value of padding-top is 32px (double the font size).

However, for some properties (those where percentages are relative to something that may require layout to determine, such as width, margin-right, text-indent, and top), percentage specified values turn into percentage computed values. [...] These relative values that remain in the computed value become absolute when the used value is determined.

---

Now an example. Let's say we have the following structure:

<div id="alpha">
  <div id="bravo">
    <div id="charlie"></div>
  </div>
</div>

... and the following CSS:

#alpha { outline: 1px solid red; width: 420px; height: 100px; }
#bravo { outline: 1px solid green; width: 50%; height: inherit; margin: 0 auto; }
#charlie { outline: 1px solid navy; width: inherit; height: inherit; margin: 0 auto; }

... you'll see this picture:

... meaning that while #charlie element has the same height as its #bravo parent, its width is 50% of its parent. Inherited was a computed value: 100px for height, 50% for width.

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While this feature might be good or bad, depending on situation, for non-fixed elements, it seems to be definitely hurting the fixed ones. As 50% value for width property is inherited as is, the used value for that dimension will be based off the viewport. It's the same with percentage-using values, such as calc(50%).

Answer 4

You have #outer as width:300px, #container as 90% means 270px, now you have given width:inherit and position:fixed that is ambiguous to browser, so use position:fixed with width:x% for #fixed