perf: preload the chunks a discussion page shows its posts with

framework/core/src/Forum/Content/Discussion.php

@@ -91,6 +91,10 @@ public function __invoke(Document $document, Request $request): Document
 
         $document->payload['apiDocument'] = $apiDocument;
 
+        // The page shows its posts only once these have loaded.
+        $document->preloadChunk('core', 'forum/components/PostStream');
+        $document->preloadChunk('core', 'forum/components/PostStreamScrubber');
+
         $document->canonicalUrl = $url([]);
         $document->page = $page;
         $document->hasNextPage = $hasNextPage;

framework/core/src/Frontend/Document.php

@@ -188,6 +188,26 @@ public function render(): string
         return $this->makeView()->render();
     }
 
+    /**
+     * Preload a JavaScript chunk the page imports as soon as it boots, so it
+     * downloads alongside the main bundle rather than after it.
+     *
+     * @param string $namespace `core`, or the ID of the extension the chunk belongs to.
+     * @param string $path The chunk's path in its JS, such as `forum/components/PostStream`.
+     */
+    public function preloadChunk(string $namespace, string $path): void
+    {
+        // The address the frontend loads it from: see ExportRegistry's versionedChunkUrl.
+        $file = "js/$namespace/$path.js";
+        $url = rtrim($this->filesystem->disk('flarum-assets')->url('js'), '/')."/$namespace/$path.js";
+
+        if ($revision = $this->versioner->getRevision($file)) {
+            $url .= "?v=$revision";
+        }
+
+        $this->preloads[] = ['href' => $url, 'as' => 'script'];
+    }
+
     protected function makeView(): View
     {
         return $this->view->make($this->appView)->with([

framework/core/tests/integration/forum/DiscussionChunkPreloadTest.php

@@ -0,0 +1,79 @@
+<?php
+
+/*
+ * This file is part of Flarum.
+ *
+ * For detailed copyright and license information, please view the
+ * LICENSE file that was distributed with this source code.
+ */
+
+namespace Flarum\Tests\integration\forum;
+
+use Flarum\Discussion\Discussion;
+use Flarum\Post\Post;
+use Flarum\Testing\integration\RetrievesAuthorizedUsers;
+use Flarum\Testing\integration\TestCase;
+use Flarum\User\User;
+use PHPUnit\Framework\Attributes\Test;
+
+class DiscussionChunkPreloadTest extends TestCase
+{
+    use RetrievesAuthorizedUsers;
+
+    protected function setUp(): void
+    {
+        parent::setUp();
+
+        $this->prepareDatabase([
+            User::class => [
+                $this->normalUser(),
+            ],
+            Discussion::class => [
+                ['id' => 1, 'title' => 'Test Discussion', 'user_id' => 1, 'first_post_id' => 1],
+            ],
+            Post::class => [
+                ['id' => 1, 'discussion_id' => 1, 'user_id' => 2, 'type' => 'comment', 'content' => '<t><p>can i haz potat?</p></t>'],
+            ],
+        ]);
+    }
+
+    private function page(string $path): string
+    {
+        $response = $this->send($this->request('GET', $path));
+
+        $this->assertEquals(200, $response->getStatusCode());
+
+        return (string) $response->getBody();
+    }
+
+    /**
+     * The address the frontend loads a chunk from (see ExportRegistry's
+     * versionedChunkUrl), worked out from what the page hands it.
+     */
+    private function chunkUrl(string $html, string $path): string
+    {
+        preg_match('/"jsChunksBaseUrl":("[^"]*")/', $html, $base);
+        preg_match('/<script id="flarum-rev-manifest" type="application\/json">(.*?)<\/script>/s', $html, $manifest);
+
+        $file = "core/$path.js";
+        $revision = json_decode($manifest[1], true)["js/$file"] ?? null;
+
+        return json_decode($base[1]).'/'.$file.($revision ? "?v=$revision" : '');
+    }
+
+    #[Test]
+    public function a_discussion_page_preloads_the_chunks_it_shows_its_posts_with()
+    {
+        $html = $this->page('/d/1');
+
+        foreach (['forum/components/PostStream', 'forum/components/PostStreamScrubber'] as $chunk) {
+            $this->assertStringContainsString('<link rel="preload" href="'.e($this->chunkUrl($html, $chunk)).'" as="script">', $html);
+        }
+    }
+
+    #[Test]
+    public function other_pages_do_not_preload_them()
+    {
+        $this->assertDoesNotMatchRegularExpression('/<link rel="preload" href="[^"]*PostStream/', $this->page('/'));
+    }
+}