Support interface BC lookup on disconnected internal boundaries

Introduce `include_internal_boundary` switch to
`side_with_boundary_id()` so we can locate boundaries even when a
neighbor exists (e.g. manually disconnected interfaces).

This fixes interface-side lookup in the temperature jump CZM-style test
without requiring AMR fallback. Updated the test to explicitly use
include_internal_boundary=true for interface IDs.

Author: ChengHauYang

include/mesh/boundary_info.h

@@ -616,12 +616,22 @@ class BoundaryInfo : public ParallelObject
   /**
    * \returns A side of element \p elem whose associated boundary id is
    * \p boundary_id if such a side exists, and \p invalid_uint otherwise.
+   * \p include_internal_boundary
+   * If true, sides on internal logical boundaries are also considered.
+   * If false (default), only sides on the geometric boundary
+   * (i.e. sides without a valid neighbor element) are inspected.
+   *
+   * Setting this to true is useful for intentionally disconnected
+   * interfaces (e.g. cohesive zone interfaces) where
+   * a neighboring element exists but the side should still be treated
+   * as a boundary for enforcement of interface conditions.
    *
    * \note If multiple sides of \p elem have the same id, only the lowest numbered
    * such side is returned.
    */
   unsigned int side_with_boundary_id(const Elem * const elem,
-                                     const boundary_id_type boundary_id) const;
+                                     const boundary_id_type boundary_id,
+                                     bool include_internal_boundary = false) const;
 
   /**
    * \returns All sides of element \p elem whose associated boundary id is

src/mesh/boundary_info.C

@@ -2298,7 +2298,8 @@ void BoundaryInfo::renumber_node_id (boundary_id_type old_id,
 
 
 unsigned int BoundaryInfo::side_with_boundary_id(const Elem * const elem,
-                                                 const boundary_id_type boundary_id_in) const
+                                                 const boundary_id_type boundary_id_in,
+                                                 bool include_internal_boundary) const
 {
   const Elem * searched_elem = elem;
 
@@ -2322,7 +2323,7 @@ unsigned int BoundaryInfo::side_with_boundary_id(const Elem * const elem,
         {
           // If we're on this external boundary then we share this
           // external boundary id
-          if (elem->neighbor_ptr(side) == nullptr)
+          if (elem->neighbor_ptr(side) == nullptr || include_internal_boundary)
             return side;
 
           // If we're on an internal boundary then we need to be sure

tests/systems/disconnected_neighbor_test.C

@@ -106,7 +106,7 @@ void assemble_temperature_jump(EquationSystems &es,
             Ke(i, j) += conductance * JxW_vol[qp] * (dphi_vol[i][qp] * dphi_vol[j][qp]);
 
       // --- Left-side interface ---
-      unsigned int side = boundary.side_with_boundary_id(elem, interface_left_id);
+      unsigned int side = boundary.side_with_boundary_id(elem, interface_left_id, true /*include_internal_boundary*/);
       if (side != libMesh::invalid_uint)
         {
           fe_face_L->reinit(elem, side);
@@ -142,7 +142,7 @@ void assemble_temperature_jump(EquationSystems &es,
         }
 
       // --- Right-side interface ---
-      side = boundary.side_with_boundary_id(elem, interface_right_id);
+      side = boundary.side_with_boundary_id(elem, interface_right_id, true /*include_internal_boundary*/);
       if (side != libMesh::invalid_uint)
         {
           fe_face_R->reinit(elem, side);
@@ -345,13 +345,16 @@ private:
     // ExodusII_IO(mesh).write_equation_systems("temperature_jump.e", es);
 
     for (Real x=0.; x<=1.; x+=0.05)
-      for (Real y=0.; y<=1.; y+=0.05)
-        {
-          Point p(x,y);
-          const Number exact = heat_exact(p,params,"","");
-          const Number approx = sys.point_value(0,p);
-          LIBMESH_ASSERT_NUMBERS_EQUAL(exact, approx, 1e-2);
-        }
+      {
+        if (std::abs(x - 0.5) < 1e-12) continue; // skip interface
+        for (Real y=0.; y<=1.; y+=0.05)
+          {
+            Point p(x,y);
+            const Number exact = heat_exact(p,params,"","");
+            const Number approx = sys.point_value(0,p);
+            LIBMESH_ASSERT_NUMBERS_EQUAL(exact, approx, 1e-2);
+          }
+      }
   }
 };