while working on
SOLR-15047, I realized I wasn't really clear on what the expected semantics of were suppose to be when "boosting"
docs that had null values in the collapse field.
I expanded on my test from that jira, to demonstrate the logic i (thought) i understood from the Ord based collector - but then discovered that depending on the group head selector used (ie: OrdScoreCollector vs OrdFieldValueCollector+OrdIntStrategy vs OrdFieldValueCollector+OrdValueSourceStrategy , etc...) you get different behavior - not just in what group head is selected, but even when the behavior should be functionally equivilent, you can get different sets of groups. (even for simple string field collapsing, independent of the bugs in numeric field collapsing).
I have not dug into WTF is happening here, but I'll attach my WIP test