Hide content underneath a transparent div while scrolling

Question

Edit with possible solution below the code

The site has a full page video.

Scroll action: not to display content which is located 250px or less from the top — so 250px of top of the video is always visible.

Perhaps a better way to understand this: hide content underneath a transparent div. But I think the first explanation is more code relevant.

Second explanation leads to numerous questions and semi-answers. None of them however solves my problem.

Here's an unanswered question that covers a lot: How to hide content that is scrolled under a transparent div?

I'd prefer the scroll bar to be full height.

Maybe this could be a hint: Add a class to a DIV when top of the window reach a specific element and remove it when not
This code could detect content position. Now to hide this upper overflow.

Demo
http://jsfiddle.net/4TgmF/

HTML

<div id="header">
    <video autoplay loop poster="https://dl.dropboxusercontent.com/u/9200106/rsz_dreamstimefree_252880.jpg" id="bgvid">
        <source src="video.mp4" type="video/mp4">
        <source src="video.ogv" type="video/ogg">
    </video>
    <div id="visible_part">Part of the background that shoud be visible at all times. This DIV and its styling is for demonstration only</div>
</div>

<div id="content">
    <p>Lorem ipsum dolor sit amet, consectetur adipiscing elit. Donec a diam lectus. Sed sit amet ipsum mauris. Maecenas congue ligula ac quam viverra nec consectetur ante hendrerit. Donec et mollis dolor. Praesent et diam eget libero egestas mattis sit amet vitae augue. Nam tincidunt congue enim, ut porta lorem lacinia consectetur. Donec ut libero sed arcu vehicula ultricies a non tortor. Lorem ipsum dolor sit amet, consectetur adipiscing elit. Aenean ut gravida lorem. Ut turpis felis, pulvinar a semper sed, adipiscing id dolor. Pellentesque auctor nisi id magna consequat sagittis. Curabitur dapibus enim sit amet elit pharetra tincidunt feugiat nisl imperdiet. Ut convallis libero in urna ultrices accumsan. Donec sed odio eros. Donec viverra mi quis quam pulvinar at malesuada arcu rhoncus. Cum sociis natoque penatibus et magnis dis parturient montes, nascetur ridiculus mus. In rutrum accumsan ultricies. Mauris vitae nisi at sem facilisis semper ac in est.</p>
</div>

CSS

* { margin:0; }
html, body {
  width:100%;
  height:100%;
}
#header {
    width:100%;
    height:100%;
}
#bgvid {
    position:fixed;
    z-index:-1000;
    width:auto;
    height:auto;
    min-width:100%;
    min-height:100%;
}
#visible_part {
    position:fixed;
    height:250px;
    border-bottom:1px solid rgba(255,255,255,0.1);
    color:#fff;
    background:rgba(0,0,0,0.1);
}
#content {
    width:100%;
    min-height:100%;
    background:#fafafa;
}

Edit
Gajus Kuizinas suggested in comments to replicate the background and use it as a mask, which doesn't really solves the problem, but he got me thinking (thanks). The key word here is mask. I found a good article on this: http://thenittygritty.co/css-masking I think this could be a great solution. The mask would have position:fixed;, top:250px;, height:100%;(-250px). The only problem is I can't figure out how to do a mask with fixed position and scrollable content. Can you see what I mean?

Answer 1

Here is a working solution in a fiddle.

Explanation

1. Place header in background
2. Set body height to header height plus content height
3. Place content in a wrapper at the bottom of the body: position: absolute; bottom: 0
4. Place content at the top of its wrapper: position: absolute; top: 0
5. Set the wrapper height to match content height
6. When the top of content wrapper is scrolled to the bottom of the visible part, change its position to fixed at that point: position: fixed; top: bottom of the visible part
7. If content wrapper is position: fixed, shift content up inside its wrapper to continue scrolling

Most of these values are set in JavaScript to get the actual calculated values, not percentages. This also allows recalculation on window resizing.

Code

HTML

<div id="header">
    <video id="bgvid" autoplay loop muted>
        <source src="http://techslides.com/demos/sample-videos/small.mp4" type="video/mp4">
        <source src="http://techslides.com/demos/sample-videos/small.ogv" type="video/ogg">
    </video>
</div>
        
<div id="content_wrapper">            
    <div id="content">
    </div>
</div>

CSS

* { 
    margin:0; 
}
html, body {
    position: relative;
    width:100%;
    height:100%;
}
#header {
    position: fixed;
    top: 0;
    left: 0;
    z-index: -1000;
    width:100%;
    height:100%;
}
#bgvid {
    width:auto;
    height:auto;
    min-width:100%;
    min-height:100%;
}
#content_wrapper {
    position: absolute;
    left: 0px;
    bottom: 0px;
    width: 100%;
    overflow: hidden;    
    z-index: -10;
}
#content {
    background: white;
    position: absolute;
    left: 0px;
    top: 0px;
}

JavaScript (where the real magic happens)

var $window = $(window);
var $body = $('body');
var $contentWrapper = $('#content_wrapper');
var $content = $('#content');
var minHeaderHeight = 250; // the height of the "visible part"

var lastWindowHeight = $window.height(); // save window height to calculate difference in height on resize

checkScroll(); // make sure scroll and all heights are correct on first load
stickyTop();   // make sure sticky top is used if needed on first load

$window.resize(function() {
    checkScroll();
    stickyTop();
});
$window.scroll(function() {
    stickyTop();
});

function checkScroll() {
    var newWindowHeight = $window.height();
    var windowHeightDif = newWindowHeight - lastWindowHeight;
    lastWindowHeight = newWindowHeight; // save current height for next call
        
    var contentHeight = $content.height();
    $contentWrapper.height(contentHeight);         // make sure wrapper will show entire content
    $body.height(newWindowHeight + contentHeight); // allow content to be scrolled off the screen by
                                                   // pushing content down by the amount of window height
    
    var windowScrollTop = $window.scrollTop();
    if (windowScrollTop > 0) {                                // don't scroll if at top to avoid video getting covered
        $window.scrollTop(windowScrollTop + windowHeightDif); // scroll by window height difference to keep content 
                                                              // in the same position on window resize
    }
}

function stickyTop() {
    var windowScrollTop = $window.scrollTop();
    var maxScroll = ($window.height() - minHeaderHeight);
    if (windowScrollTop >= maxScroll) {
        $contentWrapper.css('position', 'fixed').css('top', minHeaderHeight); // stop wrapper top at bottom of header
    } else {
        $contentWrapper.css('position', 'absolute').css('top', ''); // allow regular scrolling
    }
    
    if ($contentWrapper.css('position') === 'fixed') {       // if wrapper is fixed,
        $content.css('top', -(windowScrollTop - maxScroll)); // continue scrolling by shifting content up
    } else {
        $content.css('top', 0); // make sure content is lined up with wrapper
    }
}