Make BKE_scene_copy fully handle all duplicate modes, deep copy included.

This greatly simplifies and unifies logic.

Also addresses T77255: full scene copy will now use same preferences
parameters as object or collection duplicate to choose which data-blocks
to copy along.

source/blender/blenkernel/intern/scene.c

@@ -851,6 +851,10 @@ Scene *BKE_scene_copy(Main *bmain, Scene *sce, eSceneCopyMethod type)
         id_us_min(&sce_copy->gpd->id);
         BKE_id_copy_ex(bmain, (ID *)sce_copy->gpd, (ID **)&sce_copy->gpd, LIB_ID_COPY_ACTIONS);
       }
+
+      /* Deep-duplicate collections and objects (using preferences' settings for which sub-data to
+       * duplicate along the object itself). */
+      BKE_collection_duplicate(bmain, NULL, sce_copy->master_collection, true, true, true);
     }
     else {
       /* Remove sequencer if not full copy */
@@ -859,9 +863,6 @@ Scene *BKE_scene_copy(Main *bmain, Scene *sce, eSceneCopyMethod type)
       BKE_sequencer_editing_free(sce_copy, true);
     }
 
-    /* NOTE: part of SCE_COPY_FULL operations
-     * are done outside of blenkernel with ED_object_single_users! */
-
     return sce_copy;
   }
 }

source/blender/editors/scene/scene_edit.c

@@ -61,13 +61,13 @@ Scene *ED_scene_add(Main *bmain, bContext *C, wmWindow *win, eSceneCopyMethod me
   else { /* different kinds of copying */
     Scene *scene_old = WM_window_get_active_scene(win);
 
-    scene_new = BKE_scene_copy(bmain, scene_old, method);
-
-    /* these can't be handled in blenkernel currently, so do them here */
+    /* We are going to deep-copy collections, objects and various object data, we need to have
+     * up-to-date obdata for that. */
     if (method == SCE_COPY_FULL) {
       ED_editors_flush_edits(bmain);
-      ED_object_single_users(bmain, scene_new, true, true);
     }
+
+    scene_new = BKE_scene_copy(bmain, scene_old, method);
   }
 
   WM_window_set_active_scene(bmain, C, win, scene_new);