View file style/themes/spaces/js_scroll.js

var scroll_page = document.getElementById("scroll_page"),
    scroll_page_place = document.getElementById("scroll_page_place"),
    scroll_page_toTop = document.getElementById("scroll_page_toTop"),
    scroll_page_toBottom = document.getElementById("scroll_page_toBottom"),
    scroll_to_top = true;

function windowWidth(){
    return document.documentElement.clientWidth;
}

function scroll_page_width(){
    var window_width = windowWidth();
    
    if (window_width>=1100){
        var new_width = ((window_width - 900) / 4);
        if (new_width < 100){
            new_width = 100;
        }
        scroll_page.style.width =  new_width + "px";
    }else{
        scroll_page.style.width = "0";
    }
}

function window_scroll_Y(){
    if (window.scrollY){
        return window.scrollY;
    }else{
        return document.documentElement.scrollTop;
    }
}

function scroll_opacity(){
    var scroll = window_scroll_Y();
    if (scroll > 100){
        
        var opacity = (scroll / 1000);
        
        if (opacity > 1){
            opacity = 1;
        }
        scroll_page_toTop.style.opacity = 1;
        scroll_page_toTop.style.filter = "progid:DXImageTransform.Microsoft.Alpha(opacity=100)";
        
        scroll_page_toBottom.style.opacity = 0;
        scroll_page_toBottom.style.filter = "progid:DXImageTransform.Microsoft.Alpha(opacity=0)";
        
        scroll_page_place.style.opacity = opacity;
        var ie_opacity = opacity * 100;
        scroll_page_place.style.filter = "progid:DXImageTransform.Microsoft.Alpha(opacity="+ie_opacity+")";
        
        scroll_page.style.cursor = "pointer";
        scroll_to_top = true;
        
    }else{
        scroll_page_toTop.style.opacity = 0;
        scroll_page_place.style.opacity = 0;
        
        scroll_page_toTop.style.filter = "progid:DXImageTransform.Microsoft.Alpha(opacity=0)";
        scroll_page_place.style.filter = "progid:DXImageTransform.Microsoft.Alpha(opacity=0)";
        
        if (scroll_to_top !== true){
            scroll_page.style.cursor = "pointer";
        }else{
            scroll_page.style.cursor = "default";
        } 
    }
}

function move_page_Y(){
        if (scroll_to_top === true){
            if (window_scroll_Y() > 100){
                scroll_to_top = window_scroll_Y();
                window.scrollTo(0,0);
                
                
                scroll_page_toBottom.style.opacity = 1;
                scroll_page_toBottom.style.filter = "progid:DXImageTransform.Microsoft.Alpha(opacity=100)";
            }
        }else{
            window.scrollTo(0,scroll_to_top);
            scroll_to_top = true;
            
            scroll_page_toBottom.style.opacity = 0;
            scroll_page_toBottom.style.filter = "progid:DXImageTransform.Microsoft.Alpha(opacity=0)";
        }
}

document.body.onresize = scroll_page_width;
window.onscroll = scroll_opacity;
scroll_page.onclick = move_page_Y;

scroll_page_width();
scroll_opacity();