fix: default search sort to latest and keep dropdown label in sync

- Search with no explicit sort now defaults to latest (updated_at desc)
  on both backend and frontend, matching the forum UX expectation.
- "All discussions" link from the search box routes to /?q=...&sort=latest
  so the sort dropdown shows "Latest" immediately after submission.
- extendDiscussionState injects sort=-lastPostedAt into API params when
  no sort is present, keeping direct-URL navigation (?q=anan) consistent
  with the backend default.
- Backend guard changed from phpSortField===null to empty($sorts), which
  is the correct sentinel since $sorts is the authoritative ES sort list.

js/src/forum/PaginatedListStates/extendDiscussionState.tsx

@@ -17,6 +17,12 @@ export default function extendDiscussionState() {
       ...params.page,
     };
 
+    // Default to latest sort when searching without an explicit sort parameter.
+    // Matches the backend default and keeps the dropdown label in sync.
+    if (!params.sort) {
+      params.sort = '-lastPostedAt';
+    }
+
     if (Array.isArray(params.include)) {
       params.include = params.include.join(',');
     }

js/src/forum/SearchSources/DiscussionsSearchSource.tsx

@@ -66,7 +66,7 @@ export default class DiscussionsSearchSource implements SearchSource {
     return [
       <li className="Dropdown-header">{app.translator.trans('core.forum.search.discussions_heading')}</li>,
       <li>
-        <LinkButton icon="fas fa-search" href={app.route('index', { q: query })}>
+        <LinkButton icon="fas fa-search" href={app.route('index', { q: query, sort: 'latest' })}>
           {app.translator.trans('core.forum.search.all_discussions_button', { query })}
         </LinkButton>
       </li>,

src/Api/Controllers/SearchController.php

@@ -103,9 +103,10 @@ class SearchController extends ListDiscussionsController
             }
         }
 
-        // Default to latest when no sort is specified — faster than relevance
+        // Default to latest when no explicit sort is requested. This lets has_child use
-        // because has_child can use score_mode:none and ES can short-circuit early.
+        // score_mode:none, which skips child scoring entirely and lets ES short-circuit
-        if ($phpSortField === null) {
+        // early on large corpora. Relevance is still available via sort=relevant if added.
+        if (empty($sorts)) {
             $needsScoring = false;
             $phpSortField = 'updated_at';
             $phpSortDir   = 'desc';
@@ -220,11 +221,12 @@ class SearchController extends ListDiscussionsController
      * Build the text-matching portion of the query.
      *
      * Discussion titles are matched directly (filtered to join_field=discussion).
-     * Post bodies are matched via has_child. When $needsScoring is true (relevance
+     * Post bodies are matched via has_child. When $needsScoring is true (no explicit
-     * sort), score_mode=sum accumulates child scores onto the parent. When false
+     * sort, i.e. relevance ordering), score_mode=sum accumulates child scores onto the
-     * (any field sort, including the default updated_at), score_mode=none skips
+     * parent so that discussions with many strongly-matching posts rank higher. When
-     * scoring entirely — ES only checks whether a matching child exists, which is
+     * false (any explicit field sort), score_mode=none skips scoring entirely — ES only
-     * significantly cheaper on large corpora.
+     * checks whether a matching child exists, which is significantly cheaper on large
+     * corpora.
      * inner_hits returns the best-matching post for use as mostRelevantPost.
      * Hidden posts are only included in matching for users with post.hide permission.
      */