    /**
     * @class Ext.grid.Scroller
     * @extends Ext.Component
     *
     * Docked in an Ext.grid.Panel, controls virtualized scrolling and synchronization
     * across different sections.
     *
     * @private
     */
    Ext.define('Ext.grid.Scroller', {
        extend: 'Ext.Component',
        alias: 'widget.gridscroller',
        weight: 110,
        cls: Ext.baseCSSPrefix + 'scroller',
        focusable: false,
        reservedSpace: 0,

        renderTpl: [
            '<div class="' + Ext.baseCSSPrefix + 'scroller-ct" id="{baseId}_ct">',
                '<div class="' + Ext.baseCSSPrefix + 'stretcher" id="{baseId}_stretch"></div>',
            '</div>'
        ],

        initComponent: function() {
            var me       = this,
                dock     = me.dock,
                cls      = Ext.baseCSSPrefix + 'scroller-vertical',
                sizeProp = 'width';

            me.offsets = {bottom: 0};
            me.scrollProp = 'scrollTop';
            me.vertical = true;

            if (dock === 'top' || dock === 'bottom') {
                cls = Ext.baseCSSPrefix + 'scroller-horizontal';
                sizeProp = 'height';
                me.scrollProp = 'scrollLeft';
                me.vertical = false;
                me.weight += 5;
            }

            me[sizeProp] = me.scrollerSize = Ext.getScrollbarSize()[sizeProp];

            me.cls += (' ' + cls);

            Ext.applyIf(me.renderSelectors, {
                stretchEl: '.' + Ext.baseCSSPrefix + 'stretcher',
                scrollEl: '.' + Ext.baseCSSPrefix + 'scroller-ct'
            });
            me.callParent();
        },

        initRenderData: function () {
            var me = this,
                ret = me.callParent(arguments) || {};

            ret.baseId = me.id;

            return ret;
        },

        afterRender: function() {
            var me = this;
            me.callParent();

            me.mon(me.scrollEl, 'scroll', me.onElScroll, me);
            Ext.cache[me.el.id].skipGarbageCollection = true;
        },

        onAdded: function(container) {
            // Capture the controlling grid Panel so that we can use it even when we are undocked, and don't have an ownerCt
            this.ownerGrid = container;
            this.callParent(arguments);
        },

        getSizeCalculation: function() {
            var me     = this,
                owner  = me.getPanel(),
                width  = 1,
                height = 1,
                view, tbl;

            if (!me.vertical) {
                // TODO: Must gravitate to a single region..
                // Horizontal scrolling only scrolls virtualized region
                var items  = owner.query('tableview'),
                    center = items[1] || items[0];

                if (!center) {
                    return false;
                }
                // center is not guaranteed to have content, such as when there
                // are zero rows in the grid/tree. We read the width from the
                // headerCt instead.
                width = center.headerCt.getFullWidth();

                if (Ext.isIEQuirks) {
                    width--;
                }
            } else {
                view = owner.down('tableview:not([lockableInjected])');
                if (!view || !view.el) {
                    return false;
                }
                tbl = view.el.child('table', true);
                if (!tbl) {
                    return false;
                }

                // needs to also account for header and scroller (if still in picture)
                // should calculate from headerCt.
                height = tbl.offsetHeight;
            }
            if (isNaN(width)) {
                width = 1;
            }
            if (isNaN(height)) {
                height = 1;
            }
            return {
                width: width,
                height: height
            };
        },

        invalidate: function(firstPass) {
            var me = this,
                stretchEl = me.stretchEl;

            if (!stretchEl || !me.ownerCt) {
                return;
            }

            var size  = me.getSizeCalculation(),
                scrollEl = me.scrollEl,
                elDom = scrollEl.dom,
                reservedSpace = me.reservedSpace,
                pos,
                extra = 5;

            if (size) {
                stretchEl.setSize(size);

                size = me.el.getSize(true);

                if (me.vertical) {
                    size.width += extra;
                    size.height -= reservedSpace;
                    pos = 'left';
                } else {
                    size.width -= reservedSpace;
                    size.height += extra;
                    pos = 'top';
                }

                scrollEl.setSize(size);
                elDom.style[pos] = (-extra) + 'px';

                // BrowserBug: IE7
                // This makes the scroller enabled, when initially rendering.
                elDom.scrollTop = elDom.scrollTop;
            }
        },

        afterComponentLayout: function() {
            this.callParent(arguments);
            this.invalidate();
        },

        restoreScrollPos: function () {
            var me = this,
                el = this.scrollEl,
                elDom = el && el.dom;

            if (me._scrollPos !== null && elDom) {
                elDom[me.scrollProp] = me._scrollPos;
                me._scrollPos = null;
            }
        },

        setReservedSpace: function (reservedSpace) {
            var me = this;
            if (me.reservedSpace !== reservedSpace) {
                me.reservedSpace = reservedSpace;
                me.invalidate();
            }
        },

        saveScrollPos: function () {
            var me = this,
                el = this.scrollEl,
                elDom = el && el.dom;

            me._scrollPos = elDom ? elDom[me.scrollProp] : null;
        },

        /**
         * Sets the scrollTop and constrains the value between 0 and max.
         * @param {Number} scrollTop
         * @return {Number} The resulting scrollTop value after being constrained
         */
        setScrollTop: function(scrollTop) {
            var el = this.scrollEl,
                elDom = el && el.dom;

            if (elDom) {
                return elDom.scrollTop = Ext.Number.constrain(scrollTop, 0, elDom.scrollHeight - elDom.clientHeight);
            }
        },

        /**
         * Sets the scrollLeft and constrains the value between 0 and max.
         * @param {Number} scrollLeft
         * @return {Number} The resulting scrollLeft value after being constrained
         */
        setScrollLeft: function(scrollLeft) {
            var el = this.scrollEl,
                elDom = el && el.dom;

            if (elDom) {
                return elDom.scrollLeft = Ext.Number.constrain(scrollLeft, 0, elDom.scrollWidth - elDom.clientWidth);
            }
        },

        /**
         * Scroll by deltaY
         * @param {Number} delta
         * @return {Number} The resulting scrollTop value
         */
        scrollByDeltaY: function(delta) {
            var el = this.scrollEl,
                elDom = el && el.dom;

            if (elDom) {
                return this.setScrollTop(elDom.scrollTop + delta);
            }
        },

        /**
         * Scroll by deltaX
         * @param {Number} delta
         * @return {Number} The resulting scrollLeft value
         */
        scrollByDeltaX: function(delta) {
            var el = this.scrollEl,
                elDom = el && el.dom;

            if (elDom) {
                return this.setScrollLeft(elDom.scrollLeft + delta);
            }
        },

        /**
         * Scroll to the top.
         */
        scrollToTop : function(){
            this.setScrollTop(0);
        },

        // synchronize the scroller with the bound gridviews
        onElScroll: function(event, target) {
            this.fireEvent('bodyscroll', event, target);
        },

        getPanel: function() {
            var me = this;
            if (!me.panel) {
                me.panel = this.up('[scrollerOwner]');
            }
            return me.panel;
        }
    });