Responding to comments in 6710b24 and incorporating suggested changes made by @supermerill from #4553 as a single function with a flag instead of two methods.

src/CMakeLists.txt

@@ -315,6 +315,7 @@ set(SLIC3R_TEST_SOURCES
     ${TESTDIR}/libslic3r/test_skirt_brim.cpp
     ${TESTDIR}/libslic3r/test_test_data.cpp
     ${TESTDIR}/libslic3r/test_trianglemesh.cpp
+    ${TESTDIR}/libslic3r/test_extrusion_entity.cpp
     ${TESTDIR}/libslic3r/test_3mf.cpp
 )
 

xs/src/libslic3r/ExtrusionEntityCollection.cpp

@@ -220,25 +220,41 @@ ExtrusionEntityCollection::items_count() const
     return count;
 }
 
-/* Returns a single vector of pointers to all non-collection items contained in this one */
+/* Returns a single vector of chained (new) pointers to all non-collection items contained in this one */
 void
-ExtrusionEntityCollection::flatten(ExtrusionEntityCollection* retval) const
+ExtrusionEntityCollection::flatten(ExtrusionEntityCollection* retval, bool preserve_ordering) const
 {
-    for (ExtrusionEntitiesPtr::const_iterator it = this->entities.begin(); it != this->entities.end(); ++it) {
-        if ((*it)->is_collection()) {
-            ExtrusionEntityCollection* collection = dynamic_cast<ExtrusionEntityCollection*>(*it);
-            retval->append(collection->flatten().entities);
-        } else {
-            retval->append(**it);
+    if (this->no_sort and preserve_ordering) {
+        /// if we want to preserve ordering and we can't sort, break out the unsorted ones first.
+        ExtrusionEntityCollection *unsortable = new ExtrusionEntityCollection(*this);
+        retval->append(*unsortable);
+        unsortable->entities.clear();
+        for (ExtrusionEntitiesPtr::const_iterator it = this->entities.begin(); it != this->entities.end(); ++it) {
+            if ((*it)->is_collection()) {
+                ExtrusionEntityCollection* collection = dynamic_cast<ExtrusionEntityCollection*>(*it);
+                collection->flatten(unsortable, preserve_ordering);
+            }
+            else {
+                unsortable->append(**it);
+            }
+        }
+    } else {
+        for (ExtrusionEntitiesPtr::const_iterator it = this->entities.begin(); it != this->entities.end(); ++it) {
+            if ((*it)->is_collection()) {
+                ExtrusionEntityCollection* collection = dynamic_cast<ExtrusionEntityCollection*>(*it);
+                retval->append(collection->flatten(preserve_ordering).entities);
+            } else {
+                retval->append(**it);
+            }
         }
     }
 }
 
 ExtrusionEntityCollection
-ExtrusionEntityCollection::flatten() const
+ExtrusionEntityCollection::flatten(bool preserve_ordering) const
 {
     ExtrusionEntityCollection coll;
-    this->flatten(&coll);
+    this->flatten(&coll, preserve_ordering);
     return coll;
 }
 

xs/src/libslic3r/ExtrusionEntityCollection.hpp

@@ -24,7 +24,7 @@ class ExtrusionEntityCollection : public ExtrusionEntity
     ExtrusionEntityCollection& operator= (const ExtrusionEntityCollection &other);
     ~ExtrusionEntityCollection();
 
-    /// Operator to convert and flatten this collection to a single vefctor of ExtrusionPaths.
+    /// Operator to convert and flatten this collection to a single vector of ExtrusionPaths.
     operator ExtrusionPaths() const;
    
     /// This particular ExtrusionEntity is a collection.
@@ -59,11 +59,14 @@ class ExtrusionEntityCollection : public ExtrusionEntity
     size_t items_count() const;
 
     /// Returns a single vector of pointers to all non-collection items contained in this one
-    void flatten(ExtrusionEntityCollection* retval) const;
+    /// \param retval a pointer to the output memory space.
+    /// \param preserve_ordering Flag to method that will flatten if and only if the underlying collection is sortable when True (default: False).
+    void flatten(ExtrusionEntityCollection* retval, bool preserve_ordering = false) const;
 
     /// Returns a flattened copy of this ExtrusionEntityCollection. That is, all of the items in its entities vector are not collections.
     /// You should be iterating over flatten().entities if you are interested in the underlying ExtrusionEntities (and don't care about hierarchy).
-    ExtrusionEntityCollection flatten() const;
+    /// \param preserve_ordering Flag to method that will flatten if and only if the underlying collection is sortable when True (default: False).
+    ExtrusionEntityCollection flatten(bool preserve_ordering = false) const;
 
 
     double min_mm3_per_mm() const;

xs/src/libslic3r/PrintGCode.cpp

@@ -665,7 +665,7 @@ PrintGCode::process_layer(size_t idx, const Layer* layer, const Points& copies)
             // the ExtrusionPath objects of a certain infill "group" (also called "surface"
             // throughout the code). We can redefine the order of such Collections but we have to
             // do each one completely at once.
-            for(auto* fill : layerm->fills.flatten().entities) {
+            for(auto* fill : layerm->fills.flatten(true).entities) {
                 if(fill->length() == 0) continue;  // this shouldn't happen but first_point() would fail
 
                 auto extruder_id = fill->is_solid_infill()