https://www.amy-cummings.com/js/navdown.js

1var maxHeight = 400;
2
3$(function(){
4
5    $(".dropdown > li").hover(function() {
6    
7         var $container = $(this),
8             $list = $container.find("ul"),
9             $anchor = $container.find("a"),
10             height = $list.height() * 1.1,       // make sure there is enough room at the bottom
11             multiplier = height / maxHeight;     // needs to move faster if list is taller
12        
13        // need to save height here so it can revert on mouseout            
14        $container.data("origHeight", $container.height());
15        
16        // so it can retain it's rollover color all the while the dropdown is open
17        $anchor.addClass("hover");
18        
19        // make sure dropdown appears directly below parent list item    
20        $list
21            .show()
22            .css({
23                paddingTop: $container.data("origHeight")
24            });
25        
26        // don't do any animation if list shorter than max
27        if (multiplier > 1) {
28            $container
29                .css({
30                    height: maxHeight,
31                    overflow: "hidden"
32                })
33                .mousemove(function(e) {
34                    var offset = $container.offset();
35                    var relativeY = ((e.pageY - offset.top) * multiplier) - ($container.data("origHeight") * multiplier);
36                    if (relativeY > $container.data("origHeight")) {
37                        $list.css("top", -relativeY + $container.data("origHeight"));
38                    };
39                });
40        }
41        
42    }, function() {
43    
44        var $el = $(this);
45        
46        // put things back to normal
47        $el
48            .height($(this).data("origHeight"))
49            .find("ul")
50            .css({ top: 0 })
51            .hide()
52            .end()
53            .find("a")
54            .removeClass("hover");
55    
56    });
57    
58
59    
60    
61});
62
63