The source code

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

        constructor: function(config){
            Ext.apply(this, config);
            Ext.list.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 handleWidth = 5,
                x = e.getPageX(),
                header = e.getTarget('em', 3, true);
            if(header){
                var region = header.getRegion(),
                    style = header.dom.style,
                    parentNode = header.dom.parentNode;

                if(x - region.left <= handleWidth && parentNode != parentNode.parentNode.firstChild){
                    this.activeHd = Ext.get(parentNode.previousSibling.firstChild);
                    style.cursor = Ext.isWebKit ? 'e-resize' : 'col-resize';
                } else if(region.right - x <= handleWidth && parentNode != parentNode.parentNode.lastChild.previousSibling){
                    this.activeHd = header;
                    style.cursor = Ext.isWebKit ? 'w-resize' : 'col-resize';
                } else{
                    delete this.activeHd;
                    style.cursor = '';
                }
            }
        },

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

        onStart: function(e){

            var me = this,
                view = me.view,
                dragHeader = me.dragHd,
                x = me.tracker.getXY()[0];

            me.proxy = view.el.createChild({cls:'x-list-resizer'});
            me.dragX = dragHeader.getX();
            me.headerIndex = view.findHeaderIndex(dragHeader);

            me.headersDisabled = view.disableHeaders;
            view.disableHeaders = true;

            me.proxy.setHeight(view.el.getHeight());
            me.proxy.setX(me.dragX);
            me.proxy.setWidth(x - me.dragX);

            this.setBoundaries();

        },

        // Sets up the boundaries for the drag/drop operation
        setBoundaries: function(relativeX){
            var view = this.view,
                headerIndex = this.headerIndex,
                width = view.innerHd.getWidth(),
                relativeX = view.innerHd.getX(),
                minWidth = Math.ceil(width * this.minPct),
                maxWidth = width - minWidth,
                numColumns = view.columns.length,
                headers = view.innerHd.select('em', true),
                minX = minWidth + relativeX,
                maxX = maxWidth + relativeX,
                header;

            if (numColumns == 2) {
                this.minX = minX;
                this.maxX = maxX;
            }else{
                header = headers.item(headerIndex + 2);
                this.minX = headers.item(headerIndex).getX() + minWidth;
                this.maxX = header ? header.getX() - minWidth : maxX;
                if (headerIndex == 0) {
                    // First
                    this.minX = minX;
                } else if (headerIndex == numColumns - 2) {
                    // Last
                    this.maxX = maxX;
                }
            }
        },

        onDrag: function(e){
            var me = this,
                cursorX = me.tracker.getXY()[0].constrain(me.minX, me.maxX);

            me.proxy.setWidth(cursorX - this.dragX);
        },

        onEnd: function(e){
            /* calculate desired width by measuring proxy and then remove it */
            var newWidth = this.proxy.getWidth(),
                index = this.headerIndex,
                view = this.view,
                columns = view.columns,
                width = view.innerHd.getWidth(),
                newPercent = Math.ceil(newWidth * view.maxColumnWidth / width) / 100,
                disabled = this.headersDisabled,
                headerCol = columns[index],
                otherCol = columns[index + 1],
                totalPercent = headerCol.width + otherCol.width;

            this.proxy.remove();

            headerCol.width = newPercent;
            otherCol.width = totalPercent - newPercent;

            delete this.dragHd;
            view.setHdWidths();
            view.refresh();

            setTimeout(function(){
                view.disableHeaders = disabled;
            }, 100);
        }
    });

    // Backwards compatibility alias
    Ext.ListView.ColumnResizer = Ext.list.ColumnResizer;