fix: let a lone core file leave its system's drop

scripts/packextras.py

@@ -20,6 +20,7 @@ from common import standalone_set as platform_standalone_set
 from common import sanitize_pack_path
 from common import size_fits
 from validation import read_from_system_dir
+from region import FILE_GROUP
 
 
 def _detect_extras_prefix(config: dict, base_dest: str) -> str:
@@ -1044,7 +1045,7 @@ def extra_region_groups(
     # Enterprise group drop its German ROM. Alone, it drops nothing and is
     # never dropped.
     destination = extra.get("destination", extra.get("name", ""))
-    return [f"{emulator}:file:{destination}"]
+    return [f"{emulator}{FILE_GROUP}{destination}"]
 
 def emulator_region_drops(
     selected: list[tuple[str, dict]],
@@ -1108,5 +1109,5 @@ def _emulator_region_group(
     if not system and len(profile_systems) == 1:
         system = profile_systems[0]
     if not system:
-        return f"{emu_name}:file:{dest}"
+        return f"{emu_name}{FILE_GROUP}{dest}"
     return f"{emu_name}:system:{system}"

scripts/region.py

@@ -266,6 +266,11 @@ def _competing_ranks(
     return ranked
 
 
+# Marks the group of a file no system names (extra_region_groups and the
+# emulator-mode grouping both build it).
+FILE_GROUP = ":file:"
+
+
 def resolve_region_drops(
     groups: dict[str, list[tuple[str, str]]],
     index: dict[str, dict],
@@ -288,7 +293,14 @@ def resolve_region_drops(
 
     keep: set[str] = set()
     drop: set[str] = set()
-    for members in groups.values():
+    for group_id, members in groups.items():
+        if FILE_GROUP in group_id:
+            # A file no system names competes with nothing: it drops nothing,
+            # and keeping it would veto the group that does decide for the
+            # same path. Dolphin's GC/EUR/IPL.bin, alone as a core extra,
+            # kept what the GameCube group dropped for --region us. A system
+            # holding one file still keeps it: a group is never emptied.
+            continue
         regional: list[tuple[int, str]] = []
         world: set[str] = set()
         untagged: set[str] = set()

tests/test_region.py

@@ -267,6 +267,18 @@ class TestResolveRegionDrops(unittest.TestCase):
     def _drops(self, groups, requested):
         return region.resolve_region_drops(groups, self.index, requested)
 
+    def test_a_lone_file_never_vetoes_its_system(self):
+        """Dolphin's GC/EUR/IPL.bin came back as a core extra alone in its
+        group, and that group's keep cancelled the drop the GameCube group
+        decided: the full pack kept what --source platform dropped."""
+        groups = {
+            "psx": [("scph5501.bin", "scph5501.bin"), ("scph5502.bin", "scph5502.bin")],
+            "core:file:scph5502.bin": [("scph5502.bin", "scph5502.bin")],
+        }
+        self.assertEqual(self._drops(groups, ["north-america"]), {"scph5502.bin"})
+        alone = {"core:file:scph5502.bin": [("scph5502.bin", "scph5502.bin")]}
+        self.assertEqual(self._drops(alone, ["north-america"]), set())
+
     def test_a_missing_match_never_displaces_a_held_file(self):
         """Grouped so the report can withdraw it, a missing file must not
         take a held one's place: a Japanese request with only the American