The source code

    /**
     * @class Ext.dd.ScrollManager
     * <p>Provides automatic scrolling of overflow regions in the page during drag operations.</p>
     * <p>The ScrollManager configs will be used as the defaults for any scroll container registered with it,
     * but you can also override most of the configs per scroll container by adding a
     * <tt>ddScrollConfig</tt> object to the target element that contains these properties: {@link #hthresh},
     * {@link #vthresh}, {@link #increment} and {@link #frequency}.  Example usage:
     * <pre><code>
    var el = Ext.get('scroll-ct');
    el.ddScrollConfig = {
        vthresh: 50,
        hthresh: -1,
        frequency: 100,
        increment: 200
    };
    Ext.dd.ScrollManager.register(el);
    </code></pre>
     * <b>Note: This class uses "Point Mode" and is untested in "Intersect Mode".</b>
     * @singleton
     */
    Ext.dd.ScrollManager = function(){
        var ddm = Ext.dd.DragDropMgr;
        var els = {};
        var dragEl = null;
        var proc = {};

        var onStop = function(e){
            dragEl = null;
            clearProc();
        };

        var triggerRefresh = function(){
            if(ddm.dragCurrent){
                 ddm.refreshCache(ddm.dragCurrent.groups);
            }
        };

        var doScroll = function(){
            if(ddm.dragCurrent){
                var dds = Ext.dd.ScrollManager;
                var inc = proc.el.ddScrollConfig ?
                          proc.el.ddScrollConfig.increment : dds.increment;
                if(!dds.animate){
                    if(proc.el.scroll(proc.dir, inc)){
                        triggerRefresh();
                    }
                }else{
                    proc.el.scroll(proc.dir, inc, true, dds.animDuration, triggerRefresh);
                }
            }
        };

        var clearProc = function(){
            if(proc.id){
                clearInterval(proc.id);
            }
            proc.id = 0;
            proc.el = null;
            proc.dir = "";
        };

        var startProc = function(el, dir){
            clearProc();
            proc.el = el;
            proc.dir = dir;
            var group = el.ddScrollConfig ? el.ddScrollConfig.ddGroup : undefined,
                freq  = (el.ddScrollConfig && el.ddScrollConfig.frequency)
                      ? el.ddScrollConfig.frequency
                      : Ext.dd.ScrollManager.frequency;

            if (group === undefined || ddm.dragCurrent.ddGroup == group) {
                proc.id = setInterval(doScroll, freq);
            }
        };

        var onFire = function(e, isDrop){
            if(isDrop || !ddm.dragCurrent){ return; }
            var dds = Ext.dd.ScrollManager;
            if(!dragEl || dragEl != ddm.dragCurrent){
                dragEl = ddm.dragCurrent;
                // refresh regions on drag start
                dds.refreshCache();
            }

            var xy = Ext.lib.Event.getXY(e);
            var pt = new Ext.lib.Point(xy[0], xy[1]);
            for(var id in els){
                var el = els[id], r = el._region;
                var c = el.ddScrollConfig ? el.ddScrollConfig : dds;
                if(r && r.contains(pt) && el.isScrollable()){
                    if(r.bottom - pt.y <= c.vthresh){
                        if(proc.el != el){
                            startProc(el, "down");
                        }
                        return;
                    }else if(r.right - pt.x <= c.hthresh){
                        if(proc.el != el){
                            startProc(el, "left");
                        }
                        return;
                    }else if(pt.y - r.top <= c.vthresh){
                        if(proc.el != el){
                            startProc(el, "up");
                        }
                        return;
                    }else if(pt.x - r.left <= c.hthresh){
                        if(proc.el != el){
                            startProc(el, "right");
                        }
                        return;
                    }
                }
            }
            clearProc();
        };

        ddm.fireEvents = ddm.fireEvents.createSequence(onFire, ddm);
        ddm.stopDrag = ddm.stopDrag.createSequence(onStop, ddm);

        return {
            /**
             * Registers new overflow element(s) to auto scroll
             * @param {Mixed/Array} el The id of or the element to be scrolled or an array of either
             */
            register : function(el){
                if(Ext.isArray(el)){
                    for(var i = 0, len = el.length; i < len; i++) {
                    	this.register(el[i]);
                    }
                }else{
                    el = Ext.get(el);
                    els[el.id] = el;
                }
            },

            /**
             * Unregisters overflow element(s) so they are no longer scrolled
             * @param {Mixed/Array} el The id of or the element to be removed or an array of either
             */
            unregister : function(el){
                if(Ext.isArray(el)){
                    for(var i = 0, len = el.length; i < len; i++) {
                    	this.unregister(el[i]);
                    }
                }else{
                    el = Ext.get(el);
                    delete els[el.id];
                }
            },

            /**
             * The number of pixels from the top or bottom edge of a container the pointer needs to be to
             * trigger scrolling (defaults to 25)
             * @type Number
             */
            vthresh : 25,
            /**
             * The number of pixels from the right or left edge of a container the pointer needs to be to
             * trigger scrolling (defaults to 25)
             * @type Number
             */
            hthresh : 25,

            /**
             * The number of pixels to scroll in each scroll increment (defaults to 100)
             * @type Number
             */
            increment : 100,

            /**
             * The frequency of scrolls in milliseconds (defaults to 500)
             * @type Number
             */
            frequency : 500,

            /**
             * True to animate the scroll (defaults to true)
             * @type Boolean
             */
            animate: true,

            /**
             * The animation duration in seconds -
             * MUST BE less than Ext.dd.ScrollManager.frequency! (defaults to .4)
             * @type Number
             */
            animDuration: .4,

            /**
             * The named drag drop {@link Ext.dd.DragSource#ddGroup group} to which this container belongs (defaults to undefined).
             * If a ddGroup is specified, then container scrolling will only occur when a dragged object is in the same ddGroup.
             * @type String
             */
            ddGroup: undefined,

            /**
             * Manually trigger a cache refresh.
             */
            refreshCache : function(){
                for(var id in els){
                    if(typeof els[id] == 'object'){ // for people extending the object prototype
                        els[id]._region = els[id].getRegion();
                    }
                }
            }
        };
    }();