Having ftypes 'grouping' and 'groupsummary'
in the same Grid breaks in updateIndexes()

Example modified from
http://docs.sencha.com/ext-js/4-1/#!...roupingSummary
Example Usage

-----------------------------

Ext.define('TestResult', {
extend: 'Ext.data.Model',
fields: ['student', 'subject', {
name: 'mark',
type: 'int'
}]
});

Ext.create('Ext.grid.Panel', {
width: 200,
height: 240,
renderTo: document.body,
features: [
{ ftype :'grouping' },
{
groupHeaderTpl: 'Subject: {name}',
ftype: 'groupingsummary'
}],
store: {
model: 'TestResult',
groupField: 'subject',
data: [{
student: 'Student 1',
subject: 'Math',
mark: 84
},{
student: 'Student 1',
subject: 'Science',
mark: 72
},{
student: 'Student 2',
subject: 'Math',
mark: 96
},{
student: 'Student 2',
subject: 'Science',
mark: 68
}]
},
columns: [{
dataIndex: 'student',
text: 'Name',
summaryType: 'count',
summaryRenderer: function(value){
return Ext.String.format('{0} student{1}', value, value !== 1 ? 's' : '');
}
}, {
dataIndex: 'mark',
text: 'Mark',
summaryType: 'average'
}]
});