jquery, window.scroll, position a banner

I want to position a banner while scrolling:

The code is

$(window).scroll(function(){
    offset = $(window).scrollTop()+80;
    var h = $(window).height();

    if(offset < 80 && offset < (h-800))
    {
        offset = 80;
    }

    $('#sidebar1').animate({top:offset}, 450);
});

The problem is the offset is dynamic开发者_StackOverflow and it changes while scrolling down, the banner goes beyond the window and while scrolling down it slides up slowly. I want a constant animation while scrolling up and down.

Is there a attribute like bottom instead of top, or is there an event other than scroll to achieve a constant scroll?

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Old post I know but if anyone is looking for a solution see the jQuery stop method, code is written below.

$(window).scroll(function(){
    offset = $(window).scrollTop()+80;
    var h = $(window).height();

    if(offset < 80 && offset < (h-800))
    {
        offset = 80;
    }
    $('#sidebar1').stop();
    $('#sidebar1').animate({top:offset}, 450);
});