fix: choose regions among the files required-only keeps

tests/test_region_required_only.py

@@ -0,0 +1,39 @@
+"""--region decides among the files --required-only keeps.
+
+The regional choice ran over optional candidates too. In Batocera's Vic20
+group the optional Japanese kernel won for --region jp, the required PAL
+and NTSC kernels were dropped as beaten, and --required-only then removed
+the optional winner: the slot shipped empty.
+"""
+
+from __future__ import annotations
+
+import sys
+import unittest
+from pathlib import Path
+
+REPO_ROOT = Path(__file__).resolve().parents[1]
+sys.path.insert(0, str(REPO_ROOT / "scripts"))
+
+
+class RequiredOnlyBeforeRegion(unittest.TestCase):
+    def test_optional_files_leave_the_groups(self):
+        from packextras import platform_region_groups
+
+        systems = {"vic20": {"files": [
+            {"name": "kernel-ntsc.bin", "destination": "Vic20/kernel-ntsc.bin", "required": True},
+            {"name": "kernel-japanese.bin", "destination": "Vic20/kernel-japanese.bin",
+             "required": False},
+        ]}}
+        groups, _ = platform_region_groups(
+            {}, systems, "emulators", None, "", {}, include_extras=False, required_only=True,
+        )
+        self.assertEqual(groups["vic20"], [("Vic20/kernel-ntsc.bin", "kernel-ntsc.bin")])
+
+    def test_every_caller_passes_it(self):
+        source = (REPO_ROOT / "scripts" / "generate_pack.py").read_text(encoding="utf-8")
+        self.assertEqual(source.count("required_only=required_only,\n        )"), 2)
+
+
+if __name__ == "__main__":
+    unittest.main()