See recent posts on this topic:
- How can I use Block Forms, Multiphenicsx, and Lagrange Multipliers to Solve Poisson's Equation with Pure Neumann Boundary Conditions? - #4 by francesco-ballarin (go to the tutorial listed in the accepted solution, and notice that the case without domain decomposition does not actually require
multiphenicsx) - Singular poisson equation in DOLFINx (click on the link in the second post)