Fix #270: Use value instead of defaultValue on <select> for Preact compat

Preact treats defaultValue as a plain HTML attribute and does not
apply it to the DOM value property on <select> elements. Use
value instead — Preact has special handling that applies value
after <option> children have been reconciled, which correctly
restores the selection on form re-render.

Retain selected on <option> elements as a secondary cue so that
even if Preact reconciles children in-place (updating rather than
replacing), the <option> nodes carry the correct selection hint.

Author: User

src/reactpy_django/forms/transforms.py

@@ -33,17 +33,22 @@ def convert_textarea_children_to_prop(vdom_tree: VdomDict) -> VdomDict:
 
 
 def set_value_prop_on_select_element(vdom_tree: VdomDict) -> VdomDict:
-    """Use the `value` prop on <select> instead of setting `selected` on <option>."""
-    # If the current tag is <select>, remove 'selected' prop from any <option> children and
-    # instead set the 'value' prop on the <select> tag.
+    """Set the `value` prop on <select> instead of relying on `selected` on <option>.
+
+    Preact applies ``value`` on ``<select>`` elements after child (``<option>``)
+    reconciliation, which correctly restores the selection after a form re-render.
+    ``defaultValue`` is treated as a plain HTML attribute by Preact (unlike React
+    which has special built-in handling for it), so ``defaultValue`` would NOT
+    restore the selection on re-render — only ``value`` works reliably.
+    """
     if vdom_tree["tagName"] == "select" and "children" in vdom_tree:
         vdom_tree.setdefault("attributes", {})
         selected_options = _find_selected_options(vdom_tree)
         multiple_choice = vdom_tree["attributes"]["multiple"] = bool(vdom_tree["attributes"].get("multiple"))
         if selected_options and not multiple_choice:
-            vdom_tree["attributes"]["defaultValue"] = selected_options[0]
+            vdom_tree["attributes"]["value"] = selected_options[0]
         if selected_options and multiple_choice:
-            vdom_tree["attributes"]["defaultValue"] = selected_options
+            vdom_tree["attributes"]["value"] = selected_options
 
     return vdom_tree