diff --git a/src/slic3r/GUI/Jobs/ArrangeJob2.hpp b/src/slic3r/GUI/Jobs/ArrangeJob2.hpp
index 31d51e9124..a4130ee4d6 100644
--- a/src/slic3r/GUI/Jobs/ArrangeJob2.hpp
+++ b/src/slic3r/GUI/Jobs/ArrangeJob2.hpp
@@ -58,9 +58,15 @@ public:
         typename decltype(std::declval<ArrangeTaskT>().process_native(
             std::declval<arr2::ArrangeTaskCtl>()))::element_type;
 
+    // All callbacks are called in the main thread.
     struct Callbacks {
+        // Task is prepared but not no processing has been initiated
         std::function<void(ArrangeTaskT &)> on_prepared;
+
+        // Task has been completed but the result is not yet written (inside finalize)
         std::function<void(ArrangeTaskT &)> on_processed;
+
+        // Task result has been written
         std::function<void(ResultType &)> on_finished;
     };
 
diff --git a/src/slic3r/GUI/Plater.cpp b/src/slic3r/GUI/Plater.cpp
index 6799b9c062..7d137698cf 100644
--- a/src/slic3r/GUI/Plater.cpp
+++ b/src/slic3r/GUI/Plater.cpp
@@ -6279,7 +6279,7 @@ void Plater::fill_bed_with_instances()
             ModelObject *model_object = prototype_mi->get_object();
             assert(model_object);
 
-//            model_object->ensure_on_bed();
+            model_object->ensure_on_bed();
 
             size_t inst_cnt = model_object->instances.size();
             if (inst_cnt == 0)
@@ -6290,19 +6290,17 @@ void Plater::fill_bed_with_instances()
             if (object_idx < 0 || object_idx >= int(model().objects.size()))
                 return;
 
-            int added_cnt = result.to_add.size();
+            update(static_cast<unsigned int>(UpdateParams::FORCE_FULL_SCREEN_REFRESH));
 
-            if (added_cnt > 0) {
-                update(static_cast<unsigned int>(UpdateParams::FORCE_FULL_SCREEN_REFRESH));
+            if (!result.to_add.empty()) {
+                auto added_cnt = result.to_add.size();
 
                 // FIXME: somebody explain why this is needed for
                 // increase_object_instances
-                if (inst_cnt == 1)
+                if (result.arranged_items.size() == 1)
                     added_cnt++;
 
-                sidebar()
-                    .obj_list()
-                    ->increase_object_instances(object_idx, size_t(added_cnt));
+                sidebar().obj_list()->increase_object_instances(object_idx, added_cnt);
             }
         };