How to make jQuery Tools Overlay fade in smoothly?

Question

I've added the minimum setup of jQuery Tools Overlay to my site. When it's opened, it hits the eye hard. I found this code snippet in the docs that shows how to create a custom animation, but it produces really weird and unpredictable results (website jumps to home directory and scrolls up when overlay is opened):

// loading animation
$.tools.overlay.addEffect("drop", function(css, done) { 

   // use Overlay API to gain access to crucial elements
   var conf = this.getConf(),
       overlay = this.getOverlay();           

   // determine initial position for the overlay
   if (conf.fixed)  {
      css.position = 'fixed';
   } else {
      css.top += $(window).scrollTop();
      css.left += $(window).scrollLeft();
      css.position = 'absolute';
   } 

   // position the overlay and show it
   overlay.css(css).show();

   // begin animating with our custom easing
   overlay.animate({ top: '+=55',  opacity: 1,  width: '+=20'}, 400, 'drop', done);

   /* closing animation */
   }, function(done) {
      this.getOverlay().animate({top:'-=55', opacity:0, width:'-=20'}, 300, 'drop', function() {
         $(this).hide();
         done.call();      
      });
   }
);

$("img[rel]").overlay({
    effect: 'drop',
    mask: '#789'
});

I just want that the overlay opens like it does already (just open, no zoom in/out, etc), but at least blend in smoothly from transparent to opaque...

Must I mess around deeper in the implementation of this to achieve that?