🐛 [DH#673] Ensure that hidden layout components are also emitted

Reported by Laurens after discussing the 'sudden inclusion' of hidden
fields in the objects API data.

Backport-of: #4123

src/openforms/formio/rendering/default.py

@@ -33,7 +33,9 @@ class ContainerMixin:
     def is_visible(self) -> bool:
         # fieldset/editgrid components do not support the showInFoo properties, so we don't use the super
         # class.
-        if self.mode == RenderModes.export:
+        # In registration mode, we need to treat layout/container nodes as visible so
+        # that their children are emitted too.
+        if self.mode in {RenderModes.export, RenderModes.registration}:
             return True
 
         # We only pass the step data, since frontend logic only has access to the current step data.

src/openforms/formio/rendering/tests/test_component_node.py

@@ -1,13 +1,14 @@
 from unittest.mock import patch
 
-from django.test import TestCase
+from django.test import TestCase, tag
 
 from openforms.submissions.rendering import Renderer, RenderModes
 from openforms.submissions.tests.factories import SubmissionFactory
 
 from ..constants import RenderConfigurationOptions
 from ..nodes import ComponentNode
 from ..registry import Registry
+from ..structured import render_json
 
 
 class FormNodeTests(TestCase):
@@ -363,6 +364,40 @@ def test_explicitly_hidden_component_not_skipped_when_registration(self):
             self.assertEqual(len(nodelist), 2)
             self.assertEqual(nodelist[0].label, "A container without visible children")
 
+    @tag("dh-673")
+    def test_visible_component_inside_hidden_fieldset_not_skipped(self):
+        submission = SubmissionFactory.from_components(
+            [
+                {
+                    "type": "fieldset",
+                    "key": "fieldset",
+                    "label": "Hidden fieldset",
+                    "hidden": True,
+                    "components": [
+                        {
+                            "type": "textfield",
+                            "key": "textfield",
+                            "label": "Text field",
+                            "hidden": False,
+                        },
+                    ],
+                }
+            ],
+        )
+
+        rendered = render_json(submission)
+
+        self.assertEqual(
+            rendered,
+            {
+                submission.steps[0].form_step.slug: {
+                    "fieldset": {
+                        "textfield": "",
+                    },
+                }
+            },
+        )
+
     def test_export_always_emits_all_nodes(self):
         renderer = Renderer(self.submission, mode=RenderModes.export, as_html=False)