Modifiers should load at object offset like in Prusaslicer (#3802)

* fix precedence errors

Fix a handful of precedence errors and 1 logic/precedence error.  None of the code will compile as intended without these changes.

* Update GUI_ObjectList.cpp

Modifiers should load at same offset as object they are created against.  Based on prusaslicer ObjectList::load_from_files where it works correctly.

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Co-authored-by: SoftFever <[email protected]>

src/slic3r/GUI/GUI_ObjectList.cpp

@@ -2030,9 +2030,10 @@ void ObjectList::load_modifier(const wxArrayString& input_files, ModelObject& mo
         ModelVolume* new_volume = model_object.add_volume(std::move(mesh), type);
         new_volume->name = boost::filesystem::path(input_file).filename().string();
 
-        // BBS: object_mesh.get_init_shift() keep the relative position
-        TriangleMesh object_mesh = model_object.volumes[0]->mesh();
-        new_volume->set_offset(new_volume->mesh().get_init_shift() - object_mesh.get_init_shift());
+        // adjust offset as prusaslicer ObjectList::load_from_files does (works) instead of BBS method
+        //// BBS: object_mesh.get_init_shift() keep the relative position
+        //TriangleMesh object_mesh = model_object.volumes[0]->mesh();
+        //new_volume->set_offset(new_volume->mesh().get_init_shift() - object_mesh.get_init_shift());
 
         // set a default extruder value, since user can't add it manually
         // BBS
@@ -2056,6 +2057,8 @@ void ObjectList::load_modifier(const wxArrayString& input_files, ModelObject& mo
             const Vec3d offset = Vec3d(instance_bb.max.x(), instance_bb.min.y(), instance_bb.min.z()) + 0.5 * mesh_bb.size() - instance_offset;
             new_volume->set_offset(inv_inst_transform * offset);
         }
+        else
+            new_volume->set_offset(new_volume->source.mesh_offset - model_object.volumes.front()->source.mesh_offset);
 
         added_volumes.push_back(new_volume);
     }