The source code

    Ext.ns('Ext.ux.grid');

    /**
     * @class Ext.ux.grid.BufferView
     * @extends Ext.grid.GridView
     * A custom GridView which renders rows on an as-needed basis.
     */
    Ext.ux.grid.BufferView = Ext.extend(Ext.grid.GridView, {
    	/**
    	 * @cfg {Number} rowHeight
    	 * The height of a row in the grid.
    	 */
    	rowHeight: 19,

    	/**
    	 * @cfg {Number} borderHeight
    	 * The combined height of border-top and border-bottom of a row.
    	 */
    	borderHeight: 2,

    	/**
    	 * @cfg {Boolean/Number} scrollDelay
    	 * The number of milliseconds before rendering rows out of the visible
    	 * viewing area. Defaults to 100. Rows will render immediately with a config
    	 * of false.
    	 */
    	scrollDelay: 100,

    	/**
    	 * @cfg {Number} cacheSize
    	 * The number of rows to look forward and backwards from the currently viewable
    	 * area.  The cache applies only to rows that have been rendered already.
    	 */
    	cacheSize: 20,

    	/**
    	 * @cfg {Number} cleanDelay
    	 * The number of milliseconds to buffer cleaning of extra rows not in the
    	 * cache.
    	 */
    	cleanDelay: 500,

    	initTemplates : function(){
    		Ext.ux.grid.BufferView.superclass.initTemplates.call(this);
    		var ts = this.templates;
    		// empty div to act as a place holder for a row
    	        ts.rowHolder = new Ext.Template(
    		        ''
    		);
    		ts.rowHolder.disableFormats = true;
    		ts.rowHolder.compile();

    		ts.rowBody = new Ext.Template(
    		        '',
    			'{cells}',
    			(this.enableRowBody ? '{body}' : ''),
    			''
    		);
    		ts.rowBody.disableFormats = true;
    		ts.rowBody.compile();
    	},

    	getStyleRowHeight : function(){
    		return Ext.isBorderBox ? (this.rowHeight + this.borderHeight) : this.rowHeight;
    	},

    	getCalculatedRowHeight : function(){
    		return this.rowHeight + this.borderHeight;
    	},

    	getVisibleRowCount : function(){
    		var rh = this.getCalculatedRowHeight();
    		var visibleHeight = this.scroller.dom.clientHeight;
    		return (visibleHeight < 1) ? 0 : Math.ceil(visibleHeight / rh);
    	},

    	getVisibleRows: function(){
    		var count = this.getVisibleRowCount();
    		var sc = this.scroller.dom.scrollTop;
    		var start = (sc == 0 ? 0 : Math.floor(sc/this.getCalculatedRowHeight())-1);
    		return {
    			first: Math.max(start, 0),
    			last: Math.min(start + count + 2, this.ds.getCount()-1)
    		};
    	},

    	doRender : function(cs, rs, ds, startRow, colCount, stripe, onlyBody){
    		var ts = this.templates, ct = ts.cell, rt = ts.row, rb = ts.rowBody, last = colCount-1;
    		var rh = this.getStyleRowHeight();
    		var vr = this.getVisibleRows();
    		var tstyle = 'width:'+this.getTotalWidth()+';height:'+rh+'px;';
    		// buffers
    		var buf = [], cb, c, p = {}, rp = {tstyle: tstyle}, r;
    		for (var j = 0, len = rs.length; j < len; j++) {
    			r = rs[j]; cb = [];
    			var rowIndex = (j+startRow);
    			var visible = rowIndex >= vr.first && rowIndex <= vr.last;
    			if (visible) {
    				for (var i = 0; i < colCount; i++) {
    					c = cs[i];
    					p.id = c.id;
    					p.css = i == 0 ? 'x-grid3-cell-first ' : (i == last ? 'x-grid3-cell-last ' : '');
    					p.attr = p.cellAttr = "";
    					p.value = c.renderer(r.data[c.name], p, r, rowIndex, i, ds);
    					p.style = c.style;
    					if (p.value == undefined || p.value === "") {
    						p.value = " ";
    					}
    					if (r.dirty && typeof r.modified[c.name] !== 'undefined') {
    						p.css += ' x-grid3-dirty-cell';
    					}
    					cb[cb.length] = ct.apply(p);
    				}
    			}
    			var alt = [];
    			if(stripe && ((rowIndex+1) % 2 == 0)){
    			    alt[0] = "x-grid3-row-alt";
    			}
    			if(r.dirty){
    			    alt[1] = " x-grid3-dirty-row";
    			}
    			rp.cols = colCount;
    			if(this.getRowClass){
    			    alt[2] = this.getRowClass(r, rowIndex, rp, ds);
    			}
    			rp.alt = alt.join(" ");
    			rp.cells = cb.join("");
    			buf[buf.length] =  !visible ? ts.rowHolder.apply(rp) : (onlyBody ? rb.apply(rp) : rt.apply(rp));
    		}
    		return buf.join("");
    	},

    	isRowRendered: function(index){
    		var row = this.getRow(index);
    		return row && row.childNodes.length > 0;
    	},

    	syncScroll: function(){
    		Ext.ux.grid.BufferView.superclass.syncScroll.apply(this, arguments);
    		this.update();
    	},

    	// a (optionally) buffered method to update contents of gridview
    	update: function(){
    		if (this.scrollDelay) {
    			if (!this.renderTask) {
    				this.renderTask = new Ext.util.DelayedTask(this.doUpdate, this);
    			}
    			this.renderTask.delay(this.scrollDelay);
    		}else{
    			this.doUpdate();
    		}
    	},

    	doUpdate: function(){
    		if (this.getVisibleRowCount() > 0) {
    			var g = this.grid, cm = g.colModel, ds = g.store;
    		        var cs = this.getColumnData();

    		        var vr = this.getVisibleRows();
    			for (var i = vr.first; i <= vr.last; i++) {
    				// if row is NOT rendered and is visible, render it
    				if(!this.isRowRendered(i)){
    					var html = this.doRender(cs, [ds.getAt(i)], ds, i, cm.getColumnCount(), g.stripeRows, true);
    					this.getRow(i).innerHTML = html;
    				}
    			}
    			this.clean();
    		}
    	},

    	// a buffered method to clean rows
    	clean : function(){
    		if(!this.cleanTask){
    			this.cleanTask = new Ext.util.DelayedTask(this.doClean, this);
    		}
    		this.cleanTask.delay(this.cleanDelay);
    	},

    	doClean: function(){
    		if (this.getVisibleRowCount() > 0) {
    			var vr = this.getVisibleRows();
    			vr.first -= this.cacheSize;
    			vr.last += this.cacheSize;

    			var i = 0, rows = this.getRows();
    			// if first is less than 0, all rows have been rendered
    			// so lets clean the end...
    			if(vr.first <= 0){
    				i = vr.last + 1;
    			}
    			for(var len = this.ds.getCount(); i < len; i++){
    				// if current row is outside of first and last and
    				// has content, update the innerHTML to nothing
    				if ((i < vr.first || i > vr.last) && rows[i].innerHTML) {
    					rows[i].innerHTML = '';
    				}
    			}
    		}
    	},

    	layout: function(){
    		Ext.ux.grid.BufferView.superclass.layout.call(this);
    		this.update();
    	}
    });