Dofmap of fem.VectorFunctionSpace vs. fem.FunctionSpace(mesh, MixedElement)

[chenyongxin]

@dokken

For a mixed element, the dofs are most usually ordered (dofs_element_1, dofs_element_2),

I observed the same thing when I increased the size of the mesh, e.g.

from dolfinx import mesh, fem
msh  = mesh.create_unit_square(MPI.COMM_WORLD, 2, 2)
FE   = FiniteElement("Lagrange", msh.ufl_cell(), 1)
ME   = MixedElement([FE, FE])
V_ME = fem.FunctionSpace(msh, ME)
_, dofmaps = V_ME.sub(0).collapse()
print(dofmaps)

It returns [0, 1, 2, 6, 8, 10, 12, 14, 16]. Except for the first triangle element, other DOFs are ordered.

I am wondering if the feature of not-entirely ordered dofmap in a mixed element could cause interpolation issue especially across two non-matching meshes.