Scroll part of content in fixed position container

I have a position: fixed div in a layout, as a sidebar. I've been asked to have part of it's content stay fixed to the top of it (internally), and the rest to scroll if it overflows the bottom of the div.

I've had a look at this answer, however the solution presented there doesn't work with position: fixed or position: absolute containers, which is a pain.

I've made a JSFiddle demonstration of my problem here. The large amount of text should ideally scroll, instead of overflowing into the bottom of the page. The height of the header can vary with content, and may be animated.

body {
    background: #eee;
    font-family: sans-serif;
}

div.sidebar {
    padding: 10px;
    border: 1px solid #ccc;
    background: #fff;
    position: fixed;
    top: 10px;
    left: 10px;
    bottom: 10px;
    width: 280px;
}
div#fixed {
    background: #76a7dc;
    padding-bottom: 10px;
    color: #fff;
}

div#scrollable {
    overlow-y: scroll;
}

<div class="sidebar">
    <div id="fixed">
        Fixed content here, can be of varying height using jQuery $.animate()        
    </div>

    <div id="scrollable">
        Scrolling content<br><br>
        Lorem ipsum dolor sit amet, consectetur adipisicing elit, sed do eiusmod
tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam,
quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo
consequat. Duis aute irure dolor in reprehenderit in voluptate velit esse
cillum dolore eu fugiat nulla pariatur. Excepteur sint occaecat cupidatat non
proident, sunt in culpa qui officia deserunt mollit anim id est laborum.
        
        Lorem ipsum dolor sit amet, consectetur adipisicing elit, sed do eiusmod
tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam,
quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo
consequat. Duis aute irure dolor in reprehenderit in voluptate velit esse
cillum dolore eu fugiat nulla pariatur. Excepteur sint occaecat cupidatat non
proident, sunt in culpa qui officia deserunt mollit anim id est laborum.
        Lorem ipsum dolor sit amet, consectetur adipisicing elit, sed do eiusmod
tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam,
quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo
consequat. Duis aute irure dolor in reprehenderit in voluptate velit esse
cillum dolore eu fugiat nulla pariatur. Excepteur sint occaecat cupidatat non
proident, sunt in culpa qui officia deserunt mollit anim id est laborum.
        Lorem ipsum dolor sit amet, consectetur adipisicing elit, sed do eiusmod
tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam,
quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo
consequat. Duis aute irure dolor in reprehenderit in voluptate velit esse
cillum dolore eu fugiat nulla pariatur. Excepteur sint occaecat cupidatat non
proident, sunt in culpa qui officia deserunt mollit anim id est laborum.
    </div>
</div>

Without a fixed header, I can simply add overflow-y: scroll to div.sidebar and I can happily scroll all it's content if it overflows the bottom of the container. However, I'm running into issues with having a fixed, variable height header at the top of the sidebar, and having any content underneath that scroll if it's too long to fit into the container.

div.sidebar must stay position: fixed, and I would very much like to do this without any hacks, as well as make it as cross browser as possible. I've attempted various things, but none of them work, and I'm unsure as to what to try from here.

How can I make a div inside a position: fixed container scroll only in the Y direction when it's content overflows the containing div, with a fixed header of varying, indeterminate height? I'd very much like to stay away from JS, but if I have to use it I will.

It seems to work if you use

div#scrollable {
    overflow-y: scroll;
    height: 100%;
}

and add padding-bottom: 60px to div.sidebar.

For example: http://jsfiddle.net/AKL35/6/

However, I am unsure why it must be 60px.

Also, you missed the f from overflow-y: scroll;

What worked for me :

div#scrollable {
    overflow-y: scroll;
    max-height: 100vh;
}

I changed scrollable div to be with absolute position, and everything works for me

div.sidebar {
    overflow: hidden;
    background-color: green;
    padding: 5px;
    position: fixed;
    right: 20px;
    width: 40%;
    top: 30px;
    padding: 20px;
    bottom: 30%;
}
div#fixed {
    background: #76a7dc;
    color: #fff;
    height: 30px;
}

div#scrollable {
    overflow-y: scroll;
    background: lightblue;

    position: absolute;
    top:55px; 
    left:20px;
    right:20px;
    bottom:10px;
}

DEMO with two scrollable divs

Actually this is better way to do that. If height: 100% is used, the content goes off the border, but when it is 95% everything is in order:

div#scrollable {
    overflow-y: scroll;
    height: 95%;
}

Set the scrollable div to have a max-size and add overflow-y: scroll; to it's properties.

Edit: trying to get the jsfiddle to work, but it's not scrolling properly. This will take some time to figure out.