fix(forms): avoid quadratic selectedcontent synchronization

moli-dom/src/native/host/collections/form.rs

@@ -189,7 +189,20 @@ impl DomHost {
     }
 
     pub fn select_selectedcontent_elements(&self, handle: DomHandle) -> Vec<DomHandle> {
-        self.dom.select_selectedcontent_elements(handle)
+        if !self.is_html_element_named(handle, "select") {
+            return Vec::new();
+        }
+        self.elements_by_tag_name_ns(
+            handle,
+            Some("http://www.w3.org/1999/xhtml"),
+            "selectedcontent",
+            false,
+        )
+        .into_iter()
+        .filter(|selectedcontent| {
+            self.selectedcontent_nearest_ancestor_select(*selectedcontent) == Some(handle)
+        })
+        .collect()
     }
 
     pub fn option_is_disabled(&self, handle: DomHandle) -> bool {

moli-dom/src/native/mod.rs

@@ -1742,9 +1742,11 @@ mod tests {
         let body = host.document_body_handle().unwrap();
 
         let select = host.create_element("select");
+        assert!(host.append_child(body, select));
+        assert!(host.select_selectedcontent_elements(select).is_empty());
+
         let button = host.create_element("button");
         let selectedcontent = host.create_element("selectedcontent");
-        assert!(host.append_child(body, select));
         assert!(host.append_child(select, button));
         assert!(host.append_child(button, selectedcontent));
         assert_eq!(
@@ -1776,6 +1778,44 @@ mod tests {
             host.select_selectedcontent_elements(select),
             vec![selectedcontent]
         );
+
+        assert!(host.remove_child(button, selectedcontent));
+        assert!(host.select_selectedcontent_elements(select).is_empty());
+        assert!(host.append_child(button, selectedcontent));
+        assert_eq!(
+            host.select_selectedcontent_elements(select),
+            vec![selectedcontent]
+        );
+    }
+
+    #[test]
+    fn selectedcontent_query_index_preserves_detached_subtree_order_and_namespace() {
+        let mut host = DomHost::from_dom(NativeDom::new_html(test_url()));
+        let select = host.create_element("select");
+        assert!(host.select_selectedcontent_elements(select).is_empty());
+
+        let first_button = host.create_element("button");
+        let first = host.create_element("selectedcontent");
+        let second_button = host.create_element("button");
+        let second = host.create_element("selectedcontent");
+        let foreign = host
+            .create_element_ns(Some("urn:selectedcontent-test"), "selectedcontent")
+            .expect("foreign selectedcontent element");
+        assert!(host.append_child(select, first_button));
+        assert!(host.append_child(first_button, first));
+        assert!(host.append_child(select, second_button));
+        assert!(host.append_child(second_button, second));
+        assert!(host.append_child(select, foreign));
+
+        assert_eq!(
+            host.select_selectedcontent_elements(select),
+            vec![first, second]
+        );
+        assert!(host.insert_before(select, second_button, Some(first_button)));
+        assert_eq!(
+            host.select_selectedcontent_elements(select),
+            vec![second, first]
+        );
     }
 
     #[test]

moli-renderer-v8/src/document_runtime/mutation_commands/selectedcontent.rs

@@ -260,6 +260,13 @@ impl DocumentRuntime {
         let Some(select) = self.dom_host.option_nearest_ancestor_select(option) else {
             return false;
         };
+        if self
+            .dom_host
+            .select_selectedcontent_elements(select)
+            .is_empty()
+        {
+            return false;
+        }
         if self
             .dom_host
             .select_selected_option_elements(select)
@@ -406,12 +413,15 @@ impl DocumentRuntime {
             return false;
         }
 
+        let targets = self.dom_host.select_selectedcontent_elements(select);
+        if targets.is_empty() {
+            return false;
+        }
         let selected_option = self
             .dom_host
             .select_selected_option_elements(select)
             .first()
             .copied();
-        let targets = self.dom_host.select_selectedcontent_elements(select);
         let mut changed = false;
         for target in targets {
             changed |= self.clone_selected_option_contents_into_selectedcontent(