The source code

    /**
     * @class Ext.ListView.ColumnResizer
     * @extends Ext.util.Observable
     * Supporting Class for Ext.ListView.
     * @constructor
     * @param {Object} config
     */
    Ext.ListView.ColumnResizer = Ext.extend(Ext.util.Observable, {
        /**
         * @cfg {Number} minPct The minimum percentage to allot for any column (defaults to .05)
         */
        minPct: .05,

        constructor: function(config){
            Ext.apply(this, config);
            Ext.ListView.ColumnResizer.superclass.constructor.call(this);
        },
        init : function(listView){
            this.view = listView;
            listView.on('render', this.initEvents, this);
        },

        initEvents : function(view){
            view.mon(view.innerHd, 'mousemove', this.handleHdMove, this);
            this.tracker = new Ext.dd.DragTracker({
                onBeforeStart: this.onBeforeStart.createDelegate(this),
                onStart: this.onStart.createDelegate(this),
                onDrag: this.onDrag.createDelegate(this),
                onEnd: this.onEnd.createDelegate(this),
                tolerance: 3,
                autoStart: 300
            });
            this.tracker.initEl(view.innerHd);
            view.on('beforedestroy', this.tracker.destroy, this.tracker);
        },

        handleHdMove : function(e, t){
            var hw = 5;
            var x = e.getPageX();
            var hd = e.getTarget('em', 3, true);
            if(hd){
                var r = hd.getRegion();
                var ss = hd.dom.style;
                var pn = hd.dom.parentNode;

                if(x - r.left <= hw && pn != pn.parentNode.firstChild){
                    this.activeHd = Ext.get(pn.previousSibling.firstChild);
    				ss.cursor = Ext.isWebKit ? 'e-resize' : 'col-resize';
                } else if(r.right - x <= hw && pn != pn.parentNode.lastChild.previousSibling){
                    this.activeHd = hd;
    				ss.cursor = Ext.isWebKit ? 'w-resize' : 'col-resize';
                } else{
                    delete this.activeHd;
                    ss.cursor = '';
                }
            }
        },

        onBeforeStart : function(e){
            this.dragHd = this.activeHd;
            return !!this.dragHd;
        },

        onStart: function(e){
            this.view.disableHeaders = true;
            this.proxy = this.view.el.createChild({cls:'x-list-resizer'});
            this.proxy.setHeight(this.view.el.getHeight());

            var x = this.tracker.getXY()[0];
            var w = this.view.innerHd.getWidth();

            this.hdX = this.dragHd.getX();
            this.hdIndex = this.view.findHeaderIndex(this.dragHd);

            this.proxy.setX(this.hdX);
            this.proxy.setWidth(x-this.hdX);

            this.minWidth = w*this.minPct;
            this.maxWidth = w - (this.minWidth*(this.view.columns.length-1-this.hdIndex));
        },

        onDrag: function(e){
            var cursorX = this.tracker.getXY()[0];
            this.proxy.setWidth((cursorX-this.hdX).constrain(this.minWidth, this.maxWidth));
        },

        onEnd: function(e){
            var nw = this.proxy.getWidth();
            this.proxy.remove();

            var index = this.hdIndex;
            var vw = this.view, cs = vw.columns, len = cs.length;
            var w = this.view.innerHd.getWidth(), minPct = this.minPct * 100;

            var pct = Math.ceil((nw*100) / w);
            var diff = cs[index].width - pct;
            var each = Math.floor(diff / (len-1-index));
            var mod = diff - (each * (len-1-index));

            for(var i = index+1; i < len; i++){
                var cw = cs[i].width + each;
                var ncw = Math.max(minPct, cw);
                if(cw != ncw){
                    mod += cw - ncw;
                }
                cs[i].width = ncw;
            }
            cs[index].width = pct;
            cs[index+1].width += mod;
            delete this.dragHd;
            this.view.setHdWidths();
            this.view.refresh();
            setTimeout(function(){
                vw.disableHeaders = false;
            }, 100);
        }
    });