scroll firing too many times and crashing chrome and opera

i wrote a program in which you drag my custom scroll bar and page scrolls accordingly, everything works fine, however while testing it i discovered that if the page is too big 20 000 pixels and you do a large scroll it will crash chrome and opera.

This is caused by window.scroll(0, whereto1); being fired too many times on drag. when i comment this line of code everything works on the large scroll obviously, except for the functionality of scrolling page while dragging.

Bellow is the relevant code, if anyone has any good idea on how to do this without crashing the browser on large pages let me know. Thank you very much for your time.

var $dragging = null;
var pageheight1 = $( document ).height();
$(document.body).on("mousemove", function(e) {
    if ($dragging) {
    var wrap = jQuery('.pvs_inner_fixedbar');
        wrapoffset = wrap.offset();
        wrapbottomborder = ( wrapoffset.top + wrap.height() ) - jQuery('.pvs_scroll_marker').height();
        var pos = e.pageY;
        if(pos < wrapoffset.top){
            pos = wrapoffset.top;
        }
        if(pos > wrapbottomborder){
            pos = wrapbottomborder;
        }
        $dragging.offset({
            top: pos
        });
        var ratio1 = pageheight1 / 400;
        var whereto1 = Math.round((pos - wrapoffset.top) * ratio1) - 100;
        window.scroll(0, whereto1); // FIRES TOO MANY TIMES :((((
    }
});


$(document.body).on("mousedown", ".pvs_scroll_marker", function (e) {
    $dragging = $(e.target);
});

$(document.body).on("mouseup", function (e) {
    $dragging = null;
});