If I open the demo grid
https://www.bryntum.com/examples/grid/paged/
and add to to the features just
group: 'firstName',
groupSummary : {
collapseToHeader : true
}
it adds an icon to the column, sorts by first name but it doesn't provide a collapsible header like in https://www.bryntum.com/examples/grid/groupsummary/
What am I missing to make it work?