How to make a flexible-height modal with fixed header - css

Question

I've created a really simple modal that allows the content to decrease or expand without running off the page - always leaving 10% margin on the top and bottom. When the page isn't tall enough to contain all the modal content, the entire modal becomes scrollable.
See jsfiddle here
Is it possible, using only CSS, to replicate this behavior but only have the modal body be scrollable, so the header is always fixed. I've tried a few things, but haven't come up with the solution yet. Making the header position: fixed almost works, I have to reposition it over the modal box and then try to add padding to the body so the content is visible under the header, which doesn't budge the scrollbars down. I always prefer to exhaust all the css alternatives before I bind some js to window resize and manually manipulate the body height.

Answer 1

This might be late, but I had to solve a similar issue of fixed header, fluid height, fluid width.
This is how I tackled the issue:
Give your elements a border-box box-sizing. Use a wrapper to center and create a bounding box. This can be a fluid one with min-width and max-width + percentages.
Give your content element an overflow-y of auto and a max-height of 100%;
Use box-sizing:border-box;
The complete code should be something like this:
* {
margin: 0;
padding: 0;
box-sizing: border-box;
}
.modal {
position: fixed;
width: 100%;
height: 100%;
background-color: rgba(0,0,0,0.8);
}
.wrap {
position: relative;
margin: 0 auto;
display: block;
width: 90%;
/* Change the max-width value on a media query breakpoint to make this example more "responsive" */
max-width: 500px;
height: 90%;
padding: 30px;
}
.modal header {
height: 30px;
padding: 0;
color: #FFF;
background-color: #007;
}
.modal .body {
background-color: #FFF;
max-height: 100%;
overflow-y: auto;
}
http://jsfiddle.net/mariomc/EhR7r/

Answer 2

Applying the max-height and overflow-y settings to .body rather than to .wrap...?
Edit 1:
Nothing's turned up so far within the constraints, which suggests either JavaScript or straying from the constraints (using % for the header height or px margins).
Edit 2:
Here's an initial demo using % for the header height. I added a px min-height to the header tag to prevent the header from almost disappearing on very small screens, at the expense of the top margin (which is reduced on very small screens).
On a screen >= 400px tall, it should work exactly as per the requirements (40px header with 10% height). If the header were reduced in height, it would support slightly-smaller screens (a 30px header with 10% height would support >= 300px screens). Here's a demo with a 30px header.
It's a clumsy solution, but it's the only one that turned up without using JavaScript.
Also, note that I added an h2 and a .content tag and moved the padding:10px; there, to avoid combining % height and padding in the same elements (which leads to a taller height than the % value specified).