🚑 Hotfix for more validation mismatches

* Fixed required edit grid inside hidden fieldset throwing validation
  errors
* Fixed validation being applied to content components (which will
  never have values)
* Fixed formio dotted paths leading to containers and respective nested
  serializers

Backport-of: #4078

src/openforms/formio/registry.py

@@ -91,7 +91,10 @@ def build_serializer_field(self, component: ComponentT) -> serializers.Field:
             DeprecationWarning,
         )
 
-        if is_layout_component(component):
+        # not considered a layout component (because it doesn't have children)
+        if component["type"] == "content":
+            required = False
+        elif is_layout_component(component):
             required = False  # they do not hold data, they can never be required
         else:
             required = (

src/openforms/formio/serializers.py

@@ -8,6 +8,7 @@
 
 from __future__ import annotations
 
+import logging
 from typing import TYPE_CHECKING, TypeAlias
 
 from glom import assign, glom
@@ -80,6 +81,26 @@ def _remove_validations_from_field(self, field: serializers.Field) -> None:
             case serializers.CharField():
                 field.allow_blank = True
 
+            case serializers.ListField():
+                field.allow_null = True
+                field.allow_empty = True
+                field.min_length = None
+                field.max_length = None
+
+    def _get_required(self) -> bool:
+        return any(field.required for field in self.fields.values())
+
+    def _set_required(self, value: bool) -> None:
+        # we need a setter because the serializers.Field.__init__ sets the initival
+        # value, but we actually derive the value via :meth:`_get_required` above
+        # dynamically based on the children, so we just ignore it.
+        logging.debug(
+            "Setting the serializer required property has no effect. "
+            "This is deliberate"
+        )
+
+    required = property(_get_required, _set_required)  # type:ignore
+
 
 def dict_to_serializer(
     fields: dict[str, FieldOrNestedFields], **kwargs

src/openforms/formio/tests/validation/test_content.py

@@ -0,0 +1,19 @@
+from django.test import SimpleTestCase
+
+from ...typing import ContentComponent
+from .helpers import validate_formio_data
+
+
+class ContentValidationTests(SimpleTestCase):
+    def test_content_ignored(self):
+        component: ContentComponent = {
+            "type": "content",
+            "key": "Helaas-5.36",
+            "label": "Nope",
+            "validate": {"required": False},
+            "html": "<p>Nope</p>",
+        }
+
+        is_valid, _ = validate_formio_data(component, {})
+
+        self.assertTrue(is_valid)

src/openforms/formio/tests/validation/test_editgrid.py

@@ -4,7 +4,7 @@
 from openforms.typing import JSONObject
 
 from ...service import build_serializer
-from ...typing import EditGridComponent
+from ...typing import EditGridComponent, FieldsetComponent
 from .helpers import validate_formio_data
 
 
@@ -183,9 +183,39 @@ def test_optional_editgrid(self):
             ],
         }
 
-        is_valid, errors = validate_formio_data(
+        is_valid, _ = validate_formio_data(
             component,
             {"optionalRepeatingGroup": []},
         )
 
         self.assertTrue(is_valid)
+
+    def test_required_but_empty_editgrid(self):
+        editgrid: EditGridComponent = {
+            "type": "editgrid",
+            "key": "requiredRepeatingGroup",
+            "label": "Required repeating group",
+            "validate": {"required": True},
+            "components": [
+                {
+                    "type": "textfield",
+                    "key": "optionalTextfield",
+                    "label": "Optional Text field",
+                    "validate": {"required": False},
+                }
+            ],
+        }
+        component: FieldsetComponent = {
+            "type": "fieldset",
+            "key": "fieldset",
+            "label": "Hidden fieldset",
+            "hidden": True,
+            "components": [editgrid],
+        }
+
+        is_valid, _ = validate_formio_data(
+            component,
+            {"requiredRepeatingGroup": []},
+        )
+
+        self.assertTrue(is_valid)

src/openforms/formio/tests/validation/test_generic.py

@@ -3,7 +3,7 @@
 from openforms.typing import JSONValue
 
 from ...typing import Component
-from .helpers import validate_formio_data
+from .helpers import extract_error, validate_formio_data
 
 
 class FallbackBehaviourTests(SimpleTestCase):
@@ -21,3 +21,17 @@ def test_unknown_component_passthrough(self):
         is_valid, _ = validate_formio_data(component, data)
 
         self.assertTrue(is_valid)
+
+    def test_nested_keys_and_fields_being_required(self):
+        component: Component = {
+            "type": "textfield",
+            "key": "nested.field",
+            "label": "Nested data",
+            "validate": {"required": True},
+        }
+
+        is_valid, errors = validate_formio_data(component, {})
+
+        self.assertFalse(is_valid)
+        error = extract_error(errors, "nested")
+        self.assertEqual(error.code, "required")