bugfix: Set gpart location to hydro generator before splitting cells.

With moving mesh, the gparts are located the centroids of the hydro particles which is not the same as their generator location (used in splitting)
Because of this, hydro part and corresponding gpart could be assigned to different cells.

src/space_split.c

@@ -725,6 +725,21 @@ void space_split_mapper(void *map_data, int num_cells, void *extra_data) {
   /* Loop over the non-empty cells */
   for (int ind = 0; ind < num_cells; ind++) {
     struct cell *c = &cells_top[local_cells_with_particles[ind]];
+    if (s->with_self_gravity) {
+#ifdef MOVING_MESH_HYDRO
+      /* Temporarily reset the gpart positions to the generator positions of
+       * their corresponding hydro particle. We need to do this in order to not
+       * mess up the splitting of this cell (gparts should go with their hydro
+       * parts). */
+      for (int i = 0; i < c->hydro.count; i++) {
+        struct part* part = &c->hydro.parts[i];
+        struct gpart* gpart = part->gpart;
+        gpart->x[0] = part->x[0];
+        gpart->x[1] = part->x[1];
+        gpart->x[2] = part->x[2];
+      }
+#endif
+    }
     space_split_recursive(s, c, NULL, NULL, NULL, NULL, NULL, tpid);
 
     if (s->with_self_gravity) {
@@ -736,6 +751,19 @@ void space_split_mapper(void *map_data, int num_cells, void *extra_data) {
       for (int n = 0; n < SELF_GRAVITY_MULTIPOLE_ORDER + 1; ++n)
         max_mpole_power[n] =
             max(max_mpole_power[n], c->grav.multipole->m_pole.power[n]);
+
+#ifdef MOVING_MESH_HYDRO
+      /* Now the cell is split and all particles are at the correct location,
+       * we can set the gpart position to the position of the *centroid* of the
+       * hydro parts again. */
+      for (int i = 0; i < c->hydro.count; i++) {
+        struct part* part = &c->hydro.parts[i];
+        struct gpart* gpart = part->gpart;
+        gpart->x[0] += part->geometry.centroid[0];
+        gpart->x[1] += part->geometry.centroid[1];
+        gpart->x[2] += part->geometry.centroid[2];
+      }
+#endif
     }
   }