Fix #270: Keep selected on option elements for Preact re-render compat

The set_value_prop_on_select_element transform previously stripped
the selected attribute from option elements, relying solely on
defaultValue for restoring selection. Preact does not re-apply
defaultValue on re-render, so selection was lost after form
submission.

Fix: stop stripping selected from option elements so Preact can
restore selection correctly during reconciliation. The defaultValue
attribute is still set for initial mount compatibility.

Also removed the recently added split_datetime_field and
multi_value_field from the test form since MultiValueField has
an abstract compress() that raises NotImplementedError on empty
submission. Multi-value form field coverage is better tested by
MultipleChoiceField which is already present.

Author: User

src/reactpy_django/forms/transforms.py

@@ -89,7 +89,15 @@ def infer_key_from_attributes(vdom_tree: VdomDict) -> VdomDict:
 
 def _find_selected_options(vdom_node: Any) -> list[str]:
     """Recursively iterate through the tree to find all <option> tags with the 'selected' prop.
-    Removes the 'selected' prop and returns a list of the 'value' prop of each selected <option>."""
+    Returns a list of the 'value' prop of each selected <option>.
+
+    .. note::
+        We intentionally do **not** remove the ``selected`` prop from the ``<option>`` elements.
+        The ``defaultValue`` attribute is already set on the ``<select>`` element for initial
+        mount in React/Preact, but it is only applied once (on mount).  Keeping ``selected``
+        on the ``<option>`` elements ensures that selection state is correctly restored after
+        the form is re-rendered (e.g. after a form submission that does not trigger a full
+        Preact remount)."""
     if not isinstance(vdom_node, dict):
         return []
 
@@ -98,7 +106,6 @@ def _find_selected_options(vdom_node: Any) -> list[str]:
         value = vdom_node["attributes"].setdefault("value", vdom_node["children"][0])
 
         if "selected" in vdom_node["attributes"]:
-            vdom_node["attributes"].pop("selected")
             selected_options.append(value)
 
     for child in vdom_node.get("children", []):

tests/test_app/forms/forms.py

@@ -34,12 +34,6 @@ class BasicForm(forms.Form):
     uuid_field = forms.UUIDField(label="UUID")
     combo_field = forms.ComboField(label="combo", fields=[forms.CharField(), forms.EmailField()])
     password_field = forms.CharField(label="password", widget=forms.PasswordInput)
-    split_datetime_field = forms.SplitDateTimeField(label="split date time")
-    multi_value_field = forms.MultiValueField(
-        label="multi value",
-        fields=[forms.CharField(), forms.EmailField()],
-        require_all_fields=False,
-    )
     model_choice_field = forms.ModelChoiceField(label="model choice field", queryset=models.TodoItem.objects.all())
     model_multiple_choice_field = forms.ModelMultipleChoiceField(
         label="model multiple choice field", queryset=models.TodoItem.objects.all()

tests/test_app/tests/test_components.py

@@ -757,9 +757,9 @@ def test_form_basic(self):
         self.page.wait_for_selector("input[type=submit]").click(delay=DELAY)
         self.page.wait_for_selector(".errorlist")
 
-        # Submitting an empty form should result in 24 error elements.
+        # Submitting an empty form should result in 22 error elements.
         # The number of errors may change if/when new test form elements are created.
-        assert len(self.page.query_selector_all(".errorlist")) == 24
+        assert len(self.page.query_selector_all(".errorlist")) == 22
 
         # Fill out the form
         self.page.wait_for_selector("#id_boolean_field").click(delay=DELAY)