Fixed update of backend after the changes introduced with b3010a817b (Do not allow objects to be placed fully below bed) and pop object on the bed when only one part is left

src/libslic3r/Model.cpp

@@ -956,9 +956,22 @@ void ModelObject::center_around_origin(bool include_modifiers)
 
 void ModelObject::ensure_on_bed(bool allow_negative_z)
 {
-    const double min_z = get_min_z();
-    if (!allow_negative_z || min_z > SINKING_Z_THRESHOLD)
-        translate_instances({ 0.0, 0.0, -min_z });
+    double z_offset = 0.0;
+
+    if (allow_negative_z) {
+        if (volumes.size() == 1)
+            z_offset = -get_min_z();
+        else {
+            const double max_z = get_max_z();
+            if (max_z < SINKING_MIN_Z_THRESHOLD)
+                z_offset = SINKING_MIN_Z_THRESHOLD - max_z;
+        }
+    }
+    else
+        z_offset = -get_min_z();
+
+    if (z_offset != 0.0)
+        translate_instances(z_offset * Vec3d::UnitZ());
 }
 
 void ModelObject::translate_instances(const Vec3d& vector)
@@ -1429,6 +1442,19 @@ double ModelObject::get_min_z() const
     }
 }
 
+double ModelObject::get_max_z() const
+{
+    if (instances.empty())
+        return 0.0;
+    else {
+        double max_z = -DBL_MAX;
+        for (size_t i = 0; i < instances.size(); ++i) {
+            max_z = std::max(max_z, get_instance_max_z(i));
+        }
+        return max_z;
+    }
+}
+
 double ModelObject::get_instance_min_z(size_t instance_idx) const
 {
     double min_z = DBL_MAX;
@@ -1450,6 +1476,27 @@ double ModelObject::get_instance_min_z(size_t instance_idx) const
     return min_z + inst->get_offset(Z);
 }
 
+double ModelObject::get_instance_max_z(size_t instance_idx) const
+{
+    double max_z = -DBL_MAX;
+
+    const ModelInstance* inst = instances[instance_idx];
+    const Transform3d& mi = inst->get_matrix(true);
+
+    for (const ModelVolume* v : volumes) {
+        if (!v->is_model_part())
+            continue;
+
+        const Transform3d mv = mi * v->get_matrix();
+        const TriangleMesh& hull = v->get_convex_hull();
+        for (const stl_facet& facet : hull.stl.facet_start)
+            for (int i = 0; i < 3; ++i)
+                max_z = std::max(max_z, (mv * facet.vertex[i].cast<double>()).z());
+    }
+
+    return max_z + inst->get_offset(Z);
+}
+
 unsigned int ModelObject::check_instances_print_volume_state(const BoundingBoxf3& print_volume)
 {
     unsigned int num_printable = 0;