Fix of Random placment of top fill throughout model #7442

For this particular model, ClipperLib numerical instability causes
one of the internal surfaces to turn into bridging surfaces
on reslicing. The issue was fixed by reverting to untyped slices if possible.

src/libslic3r/PrintObject.cpp

@@ -227,6 +227,17 @@ void PrintObject::prepare_infill()
 
     m_print->set_status(30, L("Preparing infill"));
 
+    if (m_typed_slices) {
+        // To improve robustness of detect_surfaces_type() when reslicing (working with typed slices), see GH issue #7442.
+        // The preceding step (perimeter generator) only modifies extra_perimeters and the extra perimeters are only used by discover_vertical_shells()
+        // with more than a single region. If this step does not use Surface::extra_perimeters or Surface::extra_perimeters is always zero, it is safe
+        // to reset to the untyped slices before re-runnning detect_surfaces_type().
+        for (Layer* layer : m_layers) {
+            layer->restore_untyped_slices_no_extra_perimeters();
+            m_print->throw_if_canceled();
+        }
+    }
+
     // This will assign a type (top/bottom/internal) to $layerm->slices.
     // Then the classifcation of $layerm->slices is transfered onto 
     // the $layerm->fill_surfaces by clipping $layerm->fill_surfaces