The source code

    // private
    // This is a support class used internally by the Grid components
    Ext.grid.SplitDragZone = function(grid, hd, hd2){
        this.grid = grid;
        this.view = grid.getView();
        this.proxy = this.view.resizeProxy;
        Ext.grid.SplitDragZone.superclass.constructor.call(this, hd,
            "gridSplitters" + this.grid.getGridEl().id, {
            dragElId : Ext.id(this.proxy.dom), resizeFrame:false
        });
        this.setHandleElId(Ext.id(hd));
        this.setOuterHandleElId(Ext.id(hd2));
        this.scroll = false;
    };
    Ext.extend(Ext.grid.SplitDragZone, Ext.dd.DDProxy, {
        fly: Ext.Element.fly,

        b4StartDrag : function(x, y){
            this.view.headersDisabled = true;
            this.proxy.setHeight(this.view.mainWrap.getHeight());
            var w = this.cm.getColumnWidth(this.cellIndex);
            var minw = Math.max(w-this.grid.minColumnWidth, 0);
            this.resetConstraints();
            this.setXConstraint(minw, 1000);
            this.setYConstraint(0, 0);
            this.minX = x - minw;
            this.maxX = x + 1000;
            this.startPos = x;
            Ext.dd.DDProxy.prototype.b4StartDrag.call(this, x, y);
        },

        handleMouseDown : function(e){
            var ev = Ext.EventObject.setEvent(e);
            var t = this.fly(ev.getTarget());
            if(t.hasClass("x-grid-split")){
                this.cellIndex = this.view.getCellIndex(t.dom);
                this.split = t.dom;
                this.cm = this.grid.colModel;
                if(this.cm.isResizable(this.cellIndex) && !this.cm.isFixed(this.cellIndex)){
                    Ext.grid.SplitDragZone.superclass.handleMouseDown.apply(this, arguments);
                }
            }
        },

        endDrag : function(e){
            this.view.headersDisabled = false;
            var endX = Math.max(this.minX, Ext.lib.Event.getPageX(e));
            var diff = endX - this.startPos;
            this.view.onColumnSplitterMoved(this.cellIndex, this.cm.getColumnWidth(this.cellIndex)+diff);
        },

        autoOffset : function(){
            this.setDelta(0,0);
        }
    });