On the issue of temporal applicability, the Court sides with the AG entirely, and I agree it should.
The Court then takes a firmly wide approach to the notion of ‘counterclaim’ in (now) Article 8(3): it is in the interests of the sound administration of justice that the special jurisdiction for counterclaims enables the parties, in the same proceedings and before the same court, to litigate all their claims against each other that have a common origin (at 37). In circumstances such as those of the main proceedings, the counterclaim for reimbursement on the ground of unjust enrichment must be regarded as arising from the leasing contract from which the lessor’s original action originated. The alleged enrichment in the amount of the sum paid in enforcement of the judgment that has since been set aside would not have taken place without that contract. (at 38).
‘Common origin’ of course is a notion which is difficult to decide in abstracto: despite the Court’s attempts to harmonise Article 8(3)’s approach, the potential for national courts to insert local approaches remain. Even the discussion of (now) Article 8(3) in the Jenard Report hinted at the provision being a difficult marriage between local civil procedure rules on the one hand and the need for European harmonisation on the other.
(Handbook of) European Private International law, 2nd ed. 2016, chapter 2, Heading 188.8.131.52.a, Heading 184.108.40.206, Heading 2.1.1