I found where the problem is :
Ext-all-debug.js in GroupView definition line 50816 :
Code:
processEvent: function(name, e){
Ext.grid.GroupingView.superclass.processEvent.call(this, name, e);
var hd = e.getTarget('.x-grid-group-hd', this.mainBody);
if(hd){
var field = this.getGroupField(),
prefix = this.getPrefix(field),
groupValue = hd.id.substring(prefix.length),
emptyRe = new RegExp('gp-' + Ext.escapeRe(field) + '--hd'); // <--- HERE IS THE PROBLEM
groupValue = groupValue.substr(0, groupValue.length - 3);
if(groupValue || emptyRe.test(hd.id)){
this.grid.fireEvent('group' + name, this.grid, field, groupValue, e);
}
if(name == 'mousedown' && e.button == 0){
this.toggleGroup(hd.parentNode);
}
}
}