Must be the time of year, apologies for adding to 'THE LIST'.
Set GROUPBY=grouplist
shows list of groupable columns (visible columns)
and selecting one puts lister in expanded grouped mode, please add a modifier that will group the selected item from the list in collapsed mode.
Set GROUPBY=grouplist
GROUPCOLLAPSE=on
and
Set GROUPBY=grouplist GROUPCOLLAPSE=on
does not work