Can you add some tests for this? And ideally if possible also contain the code in one module or class. It looks otherwise bit fragile/hacked.
I've also tried to use it and saw eg
Not sure if it is normal to see a dash before others in Acquisition => Websites report? (maybe isn't related to this PR)
Also my understanding was we would not only always show it in two columns but maybe have an option or two different "flattening" features whether you want it grouped or not? Personally in most cases I wouldn't like this view and prefer it in one column.