How can I make a div 100% of window height?

Question

I just want to have a sidebar that will be 100% of window height, but nothing works except this:

#sidebarBack {
background: rgba(20, 20, 20, .3);
position: fixed;
width: 250px;
height: 100%;
left: 0;
}

I don't want to have position: fixed, because I have horizontally scrollable content, so a fixed part would remain, well, fixed.

Is there any way to do such a thing, maybe with relative or absolute position?

Here's an quick Fiddle just for a test and explanation: JSFiddle

Answer 1

You can use the new and so-useful-I-can't-imagine-what-took-W3C-so-long vh CSS unit:

height:100vh;

Answer 2

tl;dr - add html, body {height:100%;} to your CSS.

---

Percentage values in CSS are inherited from some ancestor that already has height declared. In your case, you need to tell all parents of your sidebar to be 100% height. I'm assuming that #sidebarBack is a direct child of body.

Essentially, your code above is telling #sidebarBack to be 100% height of its parent. Its parent (we are assuming) is body, so you need to set height: 100%; on body as well. We can't stop there, however; body inherits height from html, so we also need to set height: 100%; on html. We can stop here, because html inherits its properties from viewport, which already has a declared height of 100%.

This also means if you end up putting the #sidebar inside another div, then that div also needs height:100%;.

Here is an Updated JSFiddle.

Changed your CSS to:

html, body {
    height:100%;
}

#sidebar {
    background: rgba(20, 20, 20, .3);
    width: 100px;
    height: 100%;
    left: 0;
    float:left;
}

section#settings {
    width:80%;   
    float:left;
    margin-left:100px;
    position:fixed;
}