Having seen your recent posts in Issues Solving Pure Neumann Problems in dolfinx - #7 by Daniel.Baker, I’d like to remark that one difference between the problem here and the one in the other post is that here you actually have two constraints:
- the integral constraint on Gamma_2
- the fact that the solution u must have the same value on all DOFs on Gamma_2
Having said that, I have no experience in how to include these constraints in dolfinx, and hence I can’t help any further.