Making JavaScript Custom ScrollBar
<p>I said <strong>&#8220;JavaScript&#8221;</strong>. I'm trying to make my own JavaScript custom scrollbar. It's almost successful. The problem is that I can't get an accurate multiplier for the scrolling speed. This is my code:</p>

<pre><code>var elem = document.getElementById('scroll-area'),
    track = elem.children[1],
    thumb = track.children[0],
    height = parseInt(elem.offsetHeight, 10),
    cntHeight = parseInt(elem.children[0].offsetHeight, 10),
    trcHeight = parseInt(track.offsetHeight, 10),
    distance = cntHeight - height,
    mean = 50, // For multiplier (go faster or slower)
    current = 0;

elem.children[0].style.top = current + "px"; // Set default `top` value as `0` for initiation
thumb.style.height = Math.round(trcHeight * height / cntHeight) + 'px'; // Set the scrollbar thumb hight

var doScroll = function (e) {

    // cross-browser wheel delta
    e = window.event || e;
    var delta = Math.max(-1, Math.min(1, (e.wheelDelta || -e.detail)));

    // (1 = scroll-up, -1 = scroll-down)
    // Always check the scroll distance, make sure that the scroll distance value will not
    // increased more than the content height and/or less than zero
    if ((delta == -1 &amp;&amp; current * mean &gt;= -distance) || (delta == 1 &amp;&amp; current * mean &lt; 0)) {
        current = current + delta;
    }

    // Move element up or down by updating the `top` value
    elem.children[0].style.top = (current * mean) + 'px';
    thumb.style.top = 0 - Math.round(trcHeight * (current * mean) / cntHeight) + 'px';

    e.preventDefault();

};

if (elem.addEventListener) {
    elem.addEventListener("mousewheel", doScroll, false);
    elem.addEventListener("DOMMouseScroll", doScroll, false);
} else {
    elem.attachEvent("onmousewheel", doScroll);
}
</code></pre>

<p>Anda the markup:</p>

<pre><code>&lt;div id="scroll-area"&gt;
    &lt;div&gt;&lt;!-- CONTENT --&gt;&lt;/div&gt;
    &lt;span class="scrollbar-track"&gt;&lt;span class="scrollbar-thumb"&gt;&lt;/span&gt;&lt;/span&gt;
&lt;/div&gt;
</code></pre>

<p>My problem is on <code>mean = 50</code>. When you scroll the container until the bottom of content, the red line that I made in the demo page should stop right at the bottom of the container, not higher than  that.</p>

<p>Anyone have an idea for the accurate result?</p>

<p><strong>PS:</strong> I also want to add a function that will enable user to scroll the content by dragging the scrollbar thumb. But I think I want to focus on this issue first. Thanks for your help.</p>

<p><strong>DEMO:</strong> <a href="http://jsfiddle.net/tovic/2B8Ye/" rel="nofollow">http://jsfiddle.net/tovic/2B8Ye/</a></p>
