Prefixless subtitle sidecar files

tests/integration/subtrack_mapping/test_cli_bundle.py

@@ -421,6 +421,59 @@ class SubtrackMappingBundleTests(unittest.TestCase):
         self.assertIn("external subtitle payload", extracted_subtitle)
         self.assertNotIn("embedded subtitle payload", extracted_subtitle)
 
+    def test_subtitle_directory_without_prefix_uses_source_basename(self):
+        source_filename = "basename_substitute.mkv"
+        subtitle_directory = self.workdir / "sidecars"
+        subtitle_directory.mkdir()
+        source_path = create_source_fixture(
+            self.workdir,
+            source_filename,
+            [
+                SourceTrackSpec(TrackType.VIDEO, identity="video-0"),
+                SourceTrackSpec(TrackType.AUDIO, identity="audio-1", language="eng"),
+                SourceTrackSpec(
+                    TrackType.SUBTITLE,
+                    identity="embedded-subtitle",
+                    language="eng",
+                    subtitle_lines=("embedded subtitle payload",),
+                ),
+            ],
+        )
+        write_vtt(
+            subtitle_directory / "basename_substitute_2_deu_DEF.vtt",
+            ("external subtitle payload",),
+        )
+
+        completed = run_ffx_convert(
+            self.workdir,
+            self.home_dir,
+            self.database_path,
+            "--video-encoder",
+            "copy",
+            "--no-pattern",
+            "--no-tmdb",
+            "--no-prompt",
+            "--no-signature",
+            "--subtitle-directory",
+            str(subtitle_directory),
+            str(source_path),
+        )
+        self.assertCompleted(completed)
+        self.assertIn("matched subtitle tracks #2", completed.stdout)
+        self.assertIn("Substituting subtitle stream #2", completed.stdout)
+
+        output_path = expected_output_path(self.workdir, source_filename)
+        subtitle_stream = [
+            stream
+            for stream in ffprobe_json(output_path)["streams"]
+            if stream["codec_type"] == "subtitle"
+        ][0]
+        self.assertEqual("deu", get_tag(subtitle_stream, "language"))
+
+        extracted_subtitle = extract_first_subtitle_text(self.workdir, output_path)
+        self.assertIn("external subtitle payload", extracted_subtitle)
+        self.assertNotIn("embedded subtitle payload", extracted_subtitle)
+
     def test_subtitle_prefix_uses_configured_base_directory_when_directory_is_omitted(self):
         source_filename = "substitute_default_s01e01.mkv"
         subtitle_prefix = "substitute_default"