Infinite Grid only 1 column Issues - extjs

Question

Here is the JS fiddle where I just copy pasted the sample example given by ExtJS and Kept only 1 column and that too locked. In this scenario we have 2 errors :
Its making continuous calls to backend till it gets last result. We
can check in fiddler that the calls are getting posted to backend
continously till the last page.
I am not able to scroll(vertical) in grid
Is it bug from ExtJS 4.2.1 Infinite Grid ? is there a solution ?
If we add 1 more column unlocked then its working fine (It will make only 1 request to backend and on scroll it will make next request depending on the scroller position)
Below is the same code :
Ext.onReady(function () {
Ext.define('ForumThread', {
extend: 'Ext.data.Model',
fields: [
'title', 'forumtitle', 'forumid', 'username', {
name: 'replycount',
type: 'int'
}, {
name: 'lastpost',
mapping: 'lastpost',
type: 'date',
dateFormat: 'timestamp'
},
'lastposter', ' ', 'threadid']
});
// create the Data Store
var store = Ext.create('Ext.data.Store', {
id: 'store',
model: 'ForumThread',
//remoteGroup: true,
// allow the grid to interact with the paging scroller by buffering
buffered: true,
leadingBufferZone: 300,
pageSize: 50,
proxy: {
// load using script tags for cross domain, if the data in on the same domain as
// this page, an Ajax proxy would be better
type: 'jsonp',
url: 'http://www.sencha.com/forum/remote_topics/index.php',
reader: {
root: 'topics',
totalProperty: 'totalCount'
}
},
autoLoad: true,
listeners: {
// This particular service cannot sort on more than one field, so if grouped, disable sorting
beforeprefetch: function (store, operation) {}
}
});
var grid = Ext.create('Ext.grid.Panel', {
width: 700,
height: 500,
collapsible: true,
title: 'ExtJS.com - Browse Forums',
store: store,
loadMask: true,
// grid columns
columns: [{
id: 'last',
text: "Last Post",
locked: true,
dataIndex: 'lastpost',
width: 130,
renderer: Ext.util.Format.dateRenderer('n/j/Y g:i A'),
sortable: true,
groupable: false
}],
renderTo: Ext.getBody()
});
});