Update bbox nan fixture

tests/fixtures/datasets.py

@@ -140,11 +140,16 @@ def valid_bboxes_dataset_in_seconds(valid_bboxes_dataset):
 
 @pytest.fixture
 def valid_bboxes_dataset_with_nan(valid_bboxes_dataset):
-    """Return a valid bboxes dataset with NaN values in the position array."""
-    # Set 3 NaN values in the position array for id_0
-    valid_bboxes_dataset.position.loc[
-        {"individual": "id_0", "time": [3, 7, 8]}
-    ] = np.nan
+    """Return a valid bboxes dataset with NaN values for some detections.
+    A missing detection is represented by NaN in the ``position``, ``shape``
+    and ``confidence`` arrays simultaneously, mirroring how real bounding box
+    data (e.g. from VIA-tracks) encodes frames where an individual is absent.
+    Here, individual ``id_0`` is missing at frames 3, 7 and 8.
+    """
+    nan_selection = {"individual": "id_0", "time": [3, 7, 8]}
+    valid_bboxes_dataset.position.loc[nan_selection] = np.nan
+    valid_bboxes_dataset.shape.loc[nan_selection] = np.nan
+    valid_bboxes_dataset.confidence.loc[nan_selection] = np.nan
     return valid_bboxes_dataset