This seems to be related to my problem ; how about dropping the question of the FunctionSpace continuity and sticking to the question of the integration measure dS ? In my case I don’t want to split my domain in two, the interior boundary doesn’t split the domain in two.
I agree with @MattMcLean that the question is not trivial, the only interior boundary example I found in this forum is here and it doesn’t seem very natural to move it to dolfinx…