Fixed manipulation of sinking multipart objects resulting in wrong object/parts positioning

src/libslic3r/Model.cpp

@@ -959,7 +959,7 @@ void ModelObject::ensure_on_bed(bool allow_negative_z)
     double z_offset = 0.0;
 
     if (allow_negative_z) {
-        if (volumes.size() == 1)
+        if (parts_count() == 1)
             z_offset = -get_min_z();
         else {
             const double max_z = get_max_z();
@@ -1127,6 +1127,15 @@ size_t ModelObject::facets_count() const
     return num;
 }
 
+size_t ModelObject::parts_count() const
+{
+    size_t num = 0;
+    for (const ModelVolume* v : this->volumes)
+        if (v->is_model_part())
+            ++num;
+    return num;
+}
+
 bool ModelObject::needed_repair() const
 {
     for (const ModelVolume *v : this->volumes)