fix: preserve handled display cabinet cases

tests/test_alarm_snapshots.py

@@ -252,6 +252,64 @@ class AlarmSnapshotTests(unittest.TestCase):
         self.assertNotEqual(encoded_frames[0].rgb, source_frame.rgb)
         self.assertNotEqual(encoded_frames[0].pixel(1, 1), (0, 0, 0))
 
+    def test_capture_alert_snapshot_only_draws_alert_event_zones(self) -> None:
+        encoded_frames: list[Frame] = []
+
+        def fake_encode(frame: Frame, timeout_seconds: float) -> bytes:
+            encoded_frames.append(frame)
+            return b"jpeg-bytes"
+
+        def fake_upload(
+            image_bytes: bytes,
+            *,
+            file_name: str,
+            object_key_hint: str,
+            settings,
+            post_json_request=None,
+            post_multipart_request=None,
+        ) -> dict[str, object]:
+            return {"status": "uploaded", "object_key": "uploads/alarms/zone-only.jpg", "file_name": file_name}
+
+        source_frame = Frame(width=30, height=20, rgb=b"\x00\x00\x00" * 600)
+        result = capture_alert_snapshot(
+            source_frame,
+            [
+                {
+                    "event": "time_alarm",
+                    "severity": "alarm",
+                    "batch_id": "batch_1",
+                    "camera_id": "cam_1",
+                    "zone_id": "1",
+                    "ts": "2026-06-09T09:00:00+00:00",
+                }
+            ],
+            {
+                "alarm_snapshot_upload": {"enabled": True},
+                "zones": [
+                    {
+                        "id": "1",
+                        "label": "区域 1",
+                        "polygon": [[0.00, 0.00], [0.45, 0.00], [0.45, 1.00], [0.00, 1.00]],
+                    },
+                    {
+                        "id": "2",
+                        "label": "区域 2",
+                        "polygon": [[0.55, 0.00], [1.00, 0.00], [1.00, 1.00], [0.55, 1.00]],
+                    },
+                ],
+                "trash": {"roi": [[0.45, 0.50], [0.55, 0.50], [0.55, 1.00], [0.45, 1.00]]},
+            },
+            now=datetime(2026, 6, 9, 9, 0, tzinfo=UTC),
+            jpeg_encoder=fake_encode,
+            uploader=fake_upload,
+        )
+
+        self.assertEqual(result["status"], "uploaded")
+        self.assertEqual(len(encoded_frames), 1)
+        self.assertNotEqual(encoded_frames[0].pixel(5, 10), (0, 0, 0))
+        self.assertEqual(encoded_frames[0].pixel(25, 10), (0, 0, 0))
+        self.assertEqual(encoded_frames[0].pixel(15, 15), (0, 0, 0))
+
 
 if __name__ == "__main__":
     unittest.main()