Case Summary Report shows incorrect number of rows

When using the Case Summary Report, in scenarios where a given case client has more than one case that meets the criteria set in the report's filters, only one of those cases is included in the report results listing.

As noted at http://civicrm.stackexchange.com/questions/10675/differences-in-results-between-find-cases-and-a-case-report it appears that this report groups the results by contact:
"The Case Summary report has a grouping on contact..."
"I don't know why it has a grouping on contact. The original one (CiviCRM 3.1?) didn't. And the UI no longer seems to even display the grouping option? You'd only want to group if you were doing something like displaying the count of cases per client. I'd say change $this->_groupBy = "GROUP BY {$this->_aliases['civicrm_c2']}.id"; to $this->_groupBy = ""; in CRM/Report/Case/Form/Summary.php in the groupBy() function and see if that fixes it." Demerit

I've tested on both 4.6 and 4.7 demo sites and the problem manifests itself on both.

I'll test Demerit's hack on my site and feed back here.