Keep User Preset Values on Extruder Variants They Don't List

A user preset stores the variant list its parent had when it was saved.
When the parent later gains variants, update_diff_values_to_child_config
matched variants by name only and left the new ones at the parent's
value, so the user's settings were silently replaced there, and a
re-save wrote the system values into the user's file.

An unmatched parent variant now takes the child's first variant of the
same extruder, the rule slicing already uses in get_config_index_base.
A child without a variant list covers the parent's first extruder. The
name match also no longer indexes the child's extruder ids when it has
none.

src/libslic3r/PrintConfig.cpp

@@ -11582,8 +11582,20 @@ void DynamicPrintConfig::update_diff_values_to_child_config(DynamicPrintConfig&
     else
         variant_index.resize(1, 0);
 
+    // A parent variant the child does not list (the parent gained it after the child was saved, or the
+    // child lists none) takes the child's first variant of the same extruder, as slicing does in
+    // get_config_index_base. Left unmatched, the parent's value would silently replace the user's.
+    auto same_extruder = [&](int i, int child_extruder_id) {
+        return cur_extruder_ids.empty() || child_extruder_id < 0 ||
+               (i < int(cur_extruder_ids.size()) && cur_extruder_ids[i] == child_extruder_id);
+    };
+
     if (target_variant_count == 0) {
-        variant_index[0] = 0;
+        // The child's one value belongs to the extruder of the parent's first variant.
+        const int child_extruder_id = cur_extruder_ids.empty() ? -1 : cur_extruder_ids[0];
+        for (int i = 0; i < int(variant_index.size()); i++)
+            if (i == 0 || same_extruder(i, child_extruder_id))
+                variant_index[i] = 0;
     }
     else if ((cur_extruder_ids.size() > 0) && cur_variant_count != cur_extruder_ids.size()){
         //should not happen
@@ -11601,13 +11613,22 @@ void DynamicPrintConfig::update_diff_values_to_child_config(DynamicPrintConfig&
             for (int j = 0; j < target_variant_count; j++)
             {
                 if ((cur_extruder_variants[i] == target_extruder_variants[j])
-                    &&(cur_extruder_ids.empty() || (cur_extruder_ids[i] == target_extruder_ids[j])))
+                    &&(cur_extruder_ids.empty() || target_extruder_ids.empty() || (cur_extruder_ids[i] == target_extruder_ids[j])))
                 {
                     variant_index[i] = j;
                     break;
                 }
             }
         }
+        for (int i = 0; i < cur_variant_count; i++) {
+            if (variant_index[i] != -1)
+                continue;
+            for (int j = 0; j < target_variant_count; j++)
+                if (same_extruder(i, target_extruder_ids.empty() ? -1 : target_extruder_ids[j])) {
+                    variant_index[i] = j;
+                    break;
+                }
+        }
     }
 
     const t_config_option_keys &keys = new_config.keys();

tests/libslic3r/test_config.cpp

@@ -421,6 +421,81 @@ SCENARIO("update_diff_values_to_child_config tolerates legacy machine-limit vect
     }
 }
 
+SCENARIO("update_diff_values_to_child_config keeps a child's values on variants it does not list",
+         "[Config][Variant]") {
+    std::set<std::string> no_keys;
+    auto variants = [](std::initializer_list<std::string> names) { return new Slic3r::ConfigOptionStrings(names); };
+
+    GIVEN("A filament parent with three variants") {
+        Slic3r::DynamicPrintConfig parent;
+        parent.set_key_value("filament_extruder_variant",
+            variants({"Direct Drive Standard", "Bowden Standard", "Direct Drive High Flow"}));
+        parent.set_deserialize_strict("nozzle_temperature", "220,220,220");
+
+        WHEN("the child was saved when the parent had only its first variant") {
+            Slic3r::DynamicPrintConfig child;
+            child.set_key_value("filament_extruder_variant", variants({"Direct Drive Standard"}));
+            child.set_deserialize_strict("nozzle_temperature", "199");
+            parent.update_diff_values_to_child_config(child, "", "filament_extruder_variant",
+                                                      Slic3r::filament_options_with_variant, no_keys);
+            THEN("the child's value applies to every variant") {
+                REQUIRE(parent.opt_serialize("nozzle_temperature") == "199,199,199");
+            }
+        }
+        WHEN("the child lists every variant, in another order") {
+            Slic3r::DynamicPrintConfig child;
+            child.set_key_value("filament_extruder_variant",
+                variants({"Bowden Standard", "Direct Drive High Flow", "Direct Drive Standard"}));
+            child.set_deserialize_strict("nozzle_temperature", "190,205,199");
+            parent.update_diff_values_to_child_config(child, "", "filament_extruder_variant",
+                                                      Slic3r::filament_options_with_variant, no_keys);
+            THEN("each variant keeps its own value") {
+                REQUIRE(parent.opt_serialize("nozzle_temperature") == "199,190,205");
+            }
+        }
+        WHEN("the child lists no variants") {
+            Slic3r::DynamicPrintConfig child;
+            child.set_deserialize_strict("nozzle_temperature", "199");
+            parent.update_diff_values_to_child_config(child, "", "filament_extruder_variant",
+                                                      Slic3r::filament_options_with_variant, no_keys);
+            THEN("the child's value applies to every variant") {
+                REQUIRE(parent.opt_serialize("nozzle_temperature") == "199,199,199");
+            }
+        }
+    }
+
+    GIVEN("A two-extruder printer parent with two variants per extruder") {
+        Slic3r::DynamicPrintConfig parent;
+        parent.set_key_value("printer_extruder_variant",
+            variants({"Direct Drive Standard", "Direct Drive High Flow", "Direct Drive Standard", "Direct Drive High Flow"}));
+        parent.set_key_value("printer_extruder_id", new Slic3r::ConfigOptionInts({1, 1, 2, 2}));
+        parent.set_deserialize_strict("retraction_length", "0.8,0.8,0.8,0.8");
+
+        WHEN("the child lists only the Standard variant of each extruder") {
+            Slic3r::DynamicPrintConfig child;
+            child.set_key_value("printer_extruder_variant", variants({"Direct Drive Standard", "Direct Drive Standard"}));
+            child.set_key_value("printer_extruder_id", new Slic3r::ConfigOptionInts({1, 2}));
+            child.set_deserialize_strict("retraction_length", "1.1,2.2");
+            parent.update_diff_values_to_child_config(child, "printer_extruder_id", "printer_extruder_variant",
+                                                      Slic3r::printer_options_with_variant_1,
+                                                      Slic3r::printer_options_with_variant_2);
+            THEN("each extruder's High Flow variant takes that extruder's value") {
+                REQUIRE(parent.opt_serialize("retraction_length") == "1.1,1.1,2.2,2.2");
+            }
+        }
+        WHEN("the child lists no variants") {
+            Slic3r::DynamicPrintConfig child;
+            child.set_deserialize_strict("retraction_length", "1.1");
+            parent.update_diff_values_to_child_config(child, "printer_extruder_id", "printer_extruder_variant",
+                                                      Slic3r::printer_options_with_variant_1,
+                                                      Slic3r::printer_options_with_variant_2);
+            THEN("only the first extruder's variants take the child's value") {
+                REQUIRE(parent.opt_serialize("retraction_length") == "1.1,1.1,0.8,0.8");
+            }
+        }
+    }
+}
+
 // SCENARIO("DynamicPrintConfig JSON serialization", "[Config]") {
 //     WHEN("DynamicPrintConfig is serialized and deserialized") {
 // 	auto now = std::chrono::high_resolution_clock::now();