sticky element

calc.md

marginLeft = - ( $container.width() / 2 - parseInt($g.css('right')) - $g.innerWidth() ) = -(450 + 200 - 156)

demo.js

$(function() {
  if ($.support.fixedPosition) {
    var $g      = $('#elem'), 
        $w      = $(window), 
        cls     = 'elem-sticky',
        padtop  = 50,
        inittop = $g.offset().top - padtop;
    $w.scroll(function(){
      $g[$w.scrollTop() > inittop ? 'addClass' : 'removeClass'](cls)
    })
  }
})

elem.css

div.elem{
  position: absolute; right: -200px; top: -38px;
}
div.elem-sticky{
  position: fixed; top:20px; left:50%; margin-left: 494px;
}

备

• 直接操纵 position、margin、left、top 会闪烁
• 切换 class 效率更高