A brief post on the judgment of the CJEU in C-561/20 United Airlines. The CJEU held that the EU flight delay compensation rules of Regulation 261/2004 apply to a flight operated by non-EU airline on behalf of EU airline, even when the delay relates to flight segment outside the EU. On the issue of international jurisdiction, the Court engages with customary international law questions, referring ia to its C-366/10 ATAA judgment which I discussed here.
The CJEU firstly  repeats that since
a principle of customary international law does not have the same degree of precision as a provision of an international agreement, judicial review must necessarily be limited to the question whether, in adopting the act in question, the institutions of the European Union made manifest errors of assessment concerning the conditions for applying such a principle
I do not think its poor view on the lucidity of customary international law is justified, however its finding that only manifest errors may lead to illegality does of course mean the CJEU does not have to worry about all the nuts and bolts of territorial jurisdiction. It suffices  that there is a close connection with the territory of the EU since the Regulation specifies that connecting flights fall within the scope of that regulation on the ground that the passengers have started their journey from an airport located in a Member State. :
The regulation applies to a long delay caused in a leg of a flight operated in a third country only in limited and clearly defined circumstances in which the flight concerned, taken as a whole, is operated from an airport located in the territory of a Member State. Such a flight and its passengers thus retain a close connection with the territory of the European Union, including for the leg of the flight operated outside the European Union.
Flights which are wholly operated in a third country or between two third countries, without any connection with EU territory .