The source code

    /**
     * @class Ext.SplitBar
     * @extends Ext.util.Observable
     * Creates draggable splitter bar functionality from two elements (element to be dragged and element to be resized).
     * <br><br>
     * Usage:
     * <pre><code>
    var split = new Ext.SplitBar("elementToDrag", "elementToSize",
                       Ext.SplitBar.HORIZONTAL, Ext.SplitBar.LEFT);
    split.setAdapter(new Ext.SplitBar.AbsoluteLayoutAdapter("container"));
    split.minSize = 100;
    split.maxSize = 600;
    split.animate = true;
    split.on('moved', splitterMoved);
    </code></pre>
     * @constructor
     * Create a new SplitBar
     * @param {Mixed} dragElement The element to be dragged and act as the SplitBar.
     * @param {Mixed} resizingElement The element to be resized based on where the SplitBar element is dragged
     * @param {Number} orientation (optional) Either Ext.SplitBar.HORIZONTAL or Ext.SplitBar.VERTICAL. (Defaults to HORIZONTAL)
     * @param {Number} placement (optional) Either Ext.SplitBar.LEFT or Ext.SplitBar.RIGHT for horizontal or
                            Ext.SplitBar.TOP or Ext.SplitBar.BOTTOM for vertical. (By default, this is determined automatically by the initial
                            position of the SplitBar).
     */
    Ext.SplitBar = function(dragElement, resizingElement, orientation, placement, existingProxy){

        /** @private */
        this.el = Ext.get(dragElement, true);
        this.el.unselectable();
        /** @private */
        this.resizingEl = Ext.get(resizingElement, true);

        /**
         * @private
         * The orientation of the split. Either Ext.SplitBar.HORIZONTAL or Ext.SplitBar.VERTICAL. (Defaults to HORIZONTAL)
         * Note: If this is changed after creating the SplitBar, the placement property must be manually updated
         * @type Number
         */
        this.orientation = orientation || Ext.SplitBar.HORIZONTAL;

        /**
         * The increment, in pixels by which to move this SplitBar. When <i>undefined</i>, the SplitBar moves smoothly.
         * @type Number
         * @property tickSize
         */
        /**
         * The minimum size of the resizing element. (Defaults to 0)
         * @type Number
         */
        this.minSize = 0;

        /**
         * The maximum size of the resizing element. (Defaults to 2000)
         * @type Number
         */
        this.maxSize = 2000;

        /**
         * Whether to animate the transition to the new size
         * @type Boolean
         */
        this.animate = false;

        /**
         * Whether to create a transparent shim that overlays the page when dragging, enables dragging across iframes.
         * @type Boolean
         */
        this.useShim = false;

        /** @private */
        this.shim = null;

        if(!existingProxy){
            /** @private */
            this.proxy = Ext.SplitBar.createProxy(this.orientation);
        }else{
            this.proxy = Ext.get(existingProxy).dom;
        }
        /** @private */
        this.dd = new Ext.dd.DDProxy(this.el.dom.id, "XSplitBars", {dragElId : this.proxy.id});

        /** @private */
        this.dd.b4StartDrag = this.onStartProxyDrag.createDelegate(this);

        /** @private */
        this.dd.endDrag = this.onEndProxyDrag.createDelegate(this);

        /** @private */
        this.dragSpecs = {};

        /**
         * @private The adapter to use to positon and resize elements
         */
        this.adapter = new Ext.SplitBar.BasicLayoutAdapter();
        this.adapter.init(this);

        if(this.orientation == Ext.SplitBar.HORIZONTAL){
            /** @ignore */
            this.placement = placement || (this.el.getX() > this.resizingEl.getX() ? Ext.SplitBar.LEFT : Ext.SplitBar.RIGHT);
            this.el.addClass("x-splitbar-h");
        }else{
            /** @ignore */
            this.placement = placement || (this.el.getY() > this.resizingEl.getY() ? Ext.SplitBar.TOP : Ext.SplitBar.BOTTOM);
            this.el.addClass("x-splitbar-v");
        }

        this.addEvents(
            /**
             * @event resize
             * Fires when the splitter is moved (alias for {@link #moved})
             * @param {Ext.SplitBar} this
             * @param {Number} newSize the new width or height
             */
            "resize",
            /**
             * @event moved
             * Fires when the splitter is moved
             * @param {Ext.SplitBar} this
             * @param {Number} newSize the new width or height
             */
            "moved",
            /**
             * @event beforeresize
             * Fires before the splitter is dragged
             * @param {Ext.SplitBar} this
             */
            "beforeresize",

            "beforeapply"
        );

        Ext.SplitBar.superclass.constructor.call(this);
    };

    Ext.extend(Ext.SplitBar, Ext.util.Observable, {
        onStartProxyDrag : function(x, y){
            this.fireEvent("beforeresize", this);
            this.overlay =  Ext.DomHelper.append(document.body,  {cls: "x-drag-overlay", html: "&#160;"}, true);
            this.overlay.unselectable();
            this.overlay.setSize(Ext.lib.Dom.getViewWidth(true), Ext.lib.Dom.getViewHeight(true));
            this.overlay.show();
            Ext.get(this.proxy).setDisplayed("block");
            var size = this.adapter.getElementSize(this);
            this.activeMinSize = this.getMinimumSize();
            this.activeMaxSize = this.getMaximumSize();
            var c1 = size - this.activeMinSize;
            var c2 = Math.max(this.activeMaxSize - size, 0);
            if(this.orientation == Ext.SplitBar.HORIZONTAL){
                this.dd.resetConstraints();
                this.dd.setXConstraint(
                    this.placement == Ext.SplitBar.LEFT ? c1 : c2,
                    this.placement == Ext.SplitBar.LEFT ? c2 : c1,
                    this.tickSize
                );
                this.dd.setYConstraint(0, 0);
            }else{
                this.dd.resetConstraints();
                this.dd.setXConstraint(0, 0);
                this.dd.setYConstraint(
                    this.placement == Ext.SplitBar.TOP ? c1 : c2,
                    this.placement == Ext.SplitBar.TOP ? c2 : c1,
                    this.tickSize
                );
             }
            this.dragSpecs.startSize = size;
            this.dragSpecs.startPoint = [x, y];
            Ext.dd.DDProxy.prototype.b4StartDrag.call(this.dd, x, y);
        },

        /**
         * @private Called after the drag operation by the DDProxy
         */
        onEndProxyDrag : function(e){
            Ext.get(this.proxy).setDisplayed(false);
            var endPoint = Ext.lib.Event.getXY(e);
            if(this.overlay){
                Ext.destroy(this.overlay);
                delete this.overlay;
            }
            var newSize;
            if(this.orientation == Ext.SplitBar.HORIZONTAL){
                newSize = this.dragSpecs.startSize +
                    (this.placement == Ext.SplitBar.LEFT ?
                        endPoint[0] - this.dragSpecs.startPoint[0] :
                        this.dragSpecs.startPoint[0] - endPoint[0]
                    );
            }else{
                newSize = this.dragSpecs.startSize +
                    (this.placement == Ext.SplitBar.TOP ?
                        endPoint[1] - this.dragSpecs.startPoint[1] :
                        this.dragSpecs.startPoint[1] - endPoint[1]
                    );
            }
            newSize = Math.min(Math.max(newSize, this.activeMinSize), this.activeMaxSize);
            if(newSize != this.dragSpecs.startSize){
                if(this.fireEvent('beforeapply', this, newSize) !== false){
                    this.adapter.setElementSize(this, newSize);
                    this.fireEvent("moved", this, newSize);
                    this.fireEvent("resize", this, newSize);
                }
            }
        },

        /**
         * Get the adapter this SplitBar uses
         * @return The adapter object
         */
        getAdapter : function(){
            return this.adapter;
        },

        /**
         * Set the adapter this SplitBar uses
         * @param {Object} adapter A SplitBar adapter object
         */
        setAdapter : function(adapter){
            this.adapter = adapter;
            this.adapter.init(this);
        },

        /**
         * Gets the minimum size for the resizing element
         * @return {Number} The minimum size
         */
        getMinimumSize : function(){
            return this.minSize;
        },

        /**
         * Sets the minimum size for the resizing element
         * @param {Number} minSize The minimum size
         */
        setMinimumSize : function(minSize){
            this.minSize = minSize;
        },

        /**
         * Gets the maximum size for the resizing element
         * @return {Number} The maximum size
         */
        getMaximumSize : function(){
            return this.maxSize;
        },

        /**
         * Sets the maximum size for the resizing element
         * @param {Number} maxSize The maximum size
         */
        setMaximumSize : function(maxSize){
            this.maxSize = maxSize;
        },

        /**
         * Sets the initialize size for the resizing element
         * @param {Number} size The initial size
         */
        setCurrentSize : function(size){
            var oldAnimate = this.animate;
            this.animate = false;
            this.adapter.setElementSize(this, size);
            this.animate = oldAnimate;
        },

        /**
         * Destroy this splitbar.
         * @param {Boolean} removeEl True to remove the element
         */
        destroy : function(removeEl){
            Ext.destroy(this.shim, Ext.get(this.proxy));
            this.dd.unreg();
            if(removeEl){
                this.el.remove();
            }
            this.purgeListeners();
        }
    });

    /**
     * @private static Create our own proxy element element. So it will be the same same size on all browsers, we won't use borders. Instead we use a background color.
     */
    Ext.SplitBar.createProxy = function(dir){
        var proxy = new Ext.Element(document.createElement("div"));
        document.body.appendChild(proxy.dom);
        proxy.unselectable();
        var cls = 'x-splitbar-proxy';
        proxy.addClass(cls + ' ' + (dir == Ext.SplitBar.HORIZONTAL ? cls +'-h' : cls + '-v'));
        return proxy.dom;
    };

    /**
     * @class Ext.SplitBar.BasicLayoutAdapter
     * Default Adapter. It assumes the splitter and resizing element are not positioned
     * elements and only gets/sets the width of the element. Generally used for table based layouts.
     */
    Ext.SplitBar.BasicLayoutAdapter = function(){
    };

    Ext.SplitBar.BasicLayoutAdapter.prototype = {
        // do nothing for now
        init : function(s){

        },
        /**
         * Called before drag operations to get the current size of the resizing element.
         * @param {Ext.SplitBar} s The SplitBar using this adapter
         */
         getElementSize : function(s){
            if(s.orientation == Ext.SplitBar.HORIZONTAL){
                return s.resizingEl.getWidth();
            }else{
                return s.resizingEl.getHeight();
            }
        },

        /**
         * Called after drag operations to set the size of the resizing element.
         * @param {Ext.SplitBar} s The SplitBar using this adapter
         * @param {Number} newSize The new size to set
         * @param {Function} onComplete A function to be invoked when resizing is complete
         */
        setElementSize : function(s, newSize, onComplete){
            if(s.orientation == Ext.SplitBar.HORIZONTAL){
                if(!s.animate){
                    s.resizingEl.setWidth(newSize);
                    if(onComplete){
                        onComplete(s, newSize);
                    }
                }else{
                    s.resizingEl.setWidth(newSize, true, .1, onComplete, 'easeOut');
                }
            }else{

                if(!s.animate){
                    s.resizingEl.setHeight(newSize);
                    if(onComplete){
                        onComplete(s, newSize);
                    }
                }else{
                    s.resizingEl.setHeight(newSize, true, .1, onComplete, 'easeOut');
                }
            }
        }
    };

    /**
     *@class Ext.SplitBar.AbsoluteLayoutAdapter
     * @extends Ext.SplitBar.BasicLayoutAdapter
     * Adapter that  moves the splitter element to align with the resized sizing element.
     * Used with an absolute positioned SplitBar.
     * @param {Mixed} container The container that wraps around the absolute positioned content. If it's
     * document.body, make sure you assign an id to the body element.
     */
    Ext.SplitBar.AbsoluteLayoutAdapter = function(container){
        this.basic = new Ext.SplitBar.BasicLayoutAdapter();
        this.container = Ext.get(container);
    };

    Ext.SplitBar.AbsoluteLayoutAdapter.prototype = {
        init : function(s){
            this.basic.init(s);
        },

        getElementSize : function(s){
            return this.basic.getElementSize(s);
        },

        setElementSize : function(s, newSize, onComplete){
            this.basic.setElementSize(s, newSize, this.moveSplitter.createDelegate(this, [s]));
        },

        moveSplitter : function(s){
            var yes = Ext.SplitBar;
            switch(s.placement){
                case yes.LEFT:
                    s.el.setX(s.resizingEl.getRight());
                    break;
                case yes.RIGHT:
                    s.el.setStyle("right", (this.container.getWidth() - s.resizingEl.getLeft()) + "px");
                    break;
                case yes.TOP:
                    s.el.setY(s.resizingEl.getBottom());
                    break;
                case yes.BOTTOM:
                    s.el.setY(s.resizingEl.getTop() - s.el.getHeight());
                    break;
            }
        }
    };

    /**
     * Orientation constant - Create a vertical SplitBar
     * @static
     * @type Number
     */
    Ext.SplitBar.VERTICAL = 1;

    /**
     * Orientation constant - Create a horizontal SplitBar
     * @static
     * @type Number
     */
    Ext.SplitBar.HORIZONTAL = 2;

    /**
     * Placement constant - The resizing element is to the left of the splitter element
     * @static
     * @type Number
     */
    Ext.SplitBar.LEFT = 1;

    /**
     * Placement constant - The resizing element is to the right of the splitter element
     * @static
     * @type Number
     */
    Ext.SplitBar.RIGHT = 2;

    /**
     * Placement constant - The resizing element is positioned above the splitter element
     * @static
     * @type Number
     */
    Ext.SplitBar.TOP = 3;

    /**
     * Placement constant - The resizing element is positioned under splitter element
     * @static
     * @type Number
     */
    Ext.SplitBar.BOTTOM = 4;