scrollSpy - offset calculation

Created by: davidgengenbach

I had an issue with the offset calculation of the items within a scrollspy element.

In the refresh function (which I had to use to recalculate the offsets) there is a calculation issue!

The offset is calculated with

$href.position().top

If the $scrollElement is already scrolled the offsets can be negative (like -200). You have to add the scrollTop() of the $scrollElement to it!

Sorry for my bad english!

refresh: function() {
    var self = this,
        $targets;

    // added
    var scrollTop = self.$scrollElement.scrollTop();

    this.offsets = $([]);
    this.targets = $([]);

    $targets = this.$body.find(this.selector).map(function() {
        var $el = $(this),
            href = $el.data('target') || $el.attr('href'),
            $href = /^#\w/.test(href) && $(href);
        return ($href && $href.length && [
            [$href.position().top, href]
        ]) || null;
    }).sort(function(a, b) {
        return a[0] - b[0];
    }).each(function() {
        self.offsets.push(this[0] + scrollTop); // added + scrollTop
        self.targets.push(this[1]);
    });
},