https://alfalfalfa.com/js/okkake1b.js?20150828

1/* http://blog.quusookagaku.com/wordpress/12799/ */
2jQuery(window).on('load',function(){
3    var contentHeight = $(document).height();
4    var windowHeight = $(window).height();
5    var main = $("#main_column");
6    var mainHeight = main.outerHeight();
7    var side = $("#sidebar_left");
8    var sideHeight = side.outerHeight();
9    var target = $("#Sticky-Left");
10    var targetHeight = target.outerHeight();
11    var header = $(".Sticky-Left-header");
12    var headerPosition = header.position();
13    var footer = $("#footer");
14    var footerHeight = footer.outerHeight();
15    $(window).scroll(function(){
16        sideHeight = side.outerHeight();
17        mainHeight = main.outerHeight();
18        if(sideHeight < mainHeight) {
19            headerPosition = header.position();
20
21            var scrollLeft = $(this).scrollLeft();
22            var targetLeft = headerPosition.left;
23            var cssLeft;
24            if(scrollLeft > 0) {
25                cssLeft =  targetLeft - scrollLeft;
26            } else {
27                cssLeft = "auto";
28            }
29
30            var scrollTop = $(this).scrollTop();
31            var targetTop = headerPosition.top;
32            if(scrollTop > targetTop) {
33                windowHeight = $(window).height();
34                contentHeight = $(document).height();
35                footerHeight = footer.outerHeight();
36                var visibleBottom = scrollTop + windowHeight;
37                if(footerHeight > contentHeight - visibleBottom) {
38                    target.css({position: "fixed", left: cssLeft, top: "auto", bottom: footerHeight - (contentHeight - visibleBottom)});
39                } else {
40                    target.css({position: "fixed", left: cssLeft, top: "50px", bottom: "auto"});
41                }
42            } else {
43                target.css({position: "static", left: cssLeft, top: "auto", bottom: "auto"});
44            }
45        }
46    });
47});