【插件功能】
当用户向上或向下滚动页面到一定位置时，目标元素开始固定定位（position:fixed），当回滚到原位置时目标元素恢复到原状态，可以定制触发滚动相对屏幕位置和触发滚动方向，兼容IE6
【插件参数】
$(".target_element").scrollFix( [ "top" | "bottom" | length(可以为负,表示相对bottom), [ "top" | "bottom" ] ]);
第一个参数： 可选，默认为"top"，当目标元素到了屏幕相对的位置时开始触发固定，可以填一个数值，如100,-200 ,负值表示相对于屏幕下方
第一个参数： 可选，默认为"top"，表示触发固定的滚动方向，"top"表示从上向下滚动时触发，"bottom"表示从下向上滚动时触发
【下载插件】
下载插件（download）
【代码示例】
$("#a").scrollFix(-200);
滚动到距离下面200px时开始固定，默认从上到下触发
$("#b").scrollFix(200,"bottom");
滚动到距离上面200px时开始固定，指定"bottom"从下到上触发
$("#c").scrollFix("top","top");
滚动到距离上面0时开始固定，指定"top"从上到下触发
$("#d").scrollFix("bottom","top");
滚动到距离下面0时开始固定，指定"bottom"从下到上触发
;(function($) { ?jQuery.fn.scrollFix = function(height, dir) { ???height = height || 0; ???height = height == "top" ? 0 : height; ???return this.each(function() { ?????if (height == "bottom") { ???????height = document.documentElement.clientHeight - this.scrollHeight; ?????} else if (height < 0) { ???????height = document.documentElement.clientHeight - this.scrollHeight + height; ?????} ?????var that = $(this), ???????oldHeight = false, ???????p, r, l = that.offset().left; ?????dir = dir == "bottom" ? dir : "top"; //默认滚动方向向下 ?????if (window.XMLHttpRequest) { //非ie6用fixed ???????function getHeight() { //>=0表示上面的滚动高度大于等于目标高度 ?????????return (document.documentElement.scrollTop || document.body.scrollTop) + height - that.offset().top; ???????} ???????$(window).scroll(function() { ?????????if (oldHeight === false) { ???????????if ((getHeight() >= 0 && dir == "top") || (getHeight() <= 0 && dir == "bottom")) { ?????????????oldHeight = that.offset().top - height; ?????????????that.css({ ???????????????position: "fixed", ???????????????top: height, ???????????????left: l ?????????????}); ???????????} ?????????} else { ???????????if (dir == "top" && (document.documentElement.scrollTop || document.body.scrollTop) < oldHeight) { ?????????????that.css({ ???????????????position: "static" ?????????????}); ?????????????oldHeight = false; ???????????} else if (dir == "bottom" && (document.documentElement.scrollTop || document.body.scrollTop) > oldHeight) { ?????????????that.css({ ???????????????position: "static" ?????????????}); ?????????????oldHeight = false; ???????????} ?????????} ???????}); ?????} else { //for ie6 ???????$(window).scroll(function() { ?????????if (oldHeight === false) { //恢复前只执行一次，减少reflow ???????????if ((getHeight() >= 0 && dir == "top") || (getHeight() <= 0 && dir == "bottom")) { ?????????????oldHeight = that.offset().top - height; ?????????????r = document.createElement("span"); ?????????????p = that[0].parentNode; ?????????????p.replaceChild(r, that[0]); ?????????????document.body.appendChild(that[0]); ?????????????that[0].style.position = "absolute"; ???????????} ?????????} else if ((dir == "top" && (document.documentElement.scrollTop || document.body.scrollTop) < oldHeight) || (dir == "bottom" && (document.documentElement.scrollTop || document.body.scrollTop) > oldHeight)) { //结束 ???????????that[0].style.position = "static"; ???????????p.replaceChild(that[0], r); ???????????r = null; ???????????oldHeight = false; ?????????} else { //滚动 ???????????that.css({ ?????????????left: l, ?????????????top: height + document.documentElement.scrollTop ???????????}) ?????????} ???????}); ?????} ???}); ?};})(jQuery);原文地址：https://www.cnblogs.com/Hodor/archive/2012/07/25/2607831.html
jQuery scrollFix滚动定位插件
原文地址：https://www.cnblogs.com/hedianzhan/p/8459911.html