Extract getPageFullTitle to separate module

Moves the getPageFullTitle function from SitePage.tsx into a dedicated title.ts file, improving code organization and reusability. Also removes the now-unused getSiteStructureTitle import.
Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

packages/gitbook/src/components/SitePage/SitePage.test.ts

@@ -0,0 +1,49 @@
+import { describe, expect, it } from 'bun:test';
+
+import type { RevisionPageDocument } from '@gitbook/api';
+
+import { getPageFullTitle } from './title';
+import type { GitBookSiteContext } from '@/lib/context';
+
+function makeContext(siteTitle: string, sectionTitle = ''): GitBookSiteContext {
+    return {
+        site: { title: siteTitle },
+        visibleSections: sectionTitle
+            ? {
+                  current: { title: sectionTitle, default: false },
+                  list: [{ object: 'site-section' }, { object: 'site-section' }],
+              }
+            : undefined,
+        visibleSiteSpaces: [],
+    } as unknown as GitBookSiteContext;
+}
+
+function makePage(title: string, tagTitle?: string): RevisionPageDocument {
+    return { title, tagTitle } as unknown as RevisionPageDocument;
+}
+
+describe('getPageFullTitle', () => {
+    it('uses tagTitle as the first segment', () => {
+        expect(getPageFullTitle(makeContext('GitBook'), makePage('Page title', 'SEO title'))).toBe(
+            'SEO title | GitBook'
+        );
+    });
+
+    it('falls back to the page title when tagTitle is absent', () => {
+        expect(getPageFullTitle(makeContext('GitBook'), makePage('Page title'))).toBe(
+            'Page title | GitBook'
+        );
+    });
+
+    it('deduplicates the section title against tagTitle', () => {
+        expect(
+            getPageFullTitle(makeContext('GitBook', 'Section'), makePage('Page title', 'Section'))
+        ).toBe('Section | GitBook');
+    });
+
+    it('deduplicates the site title against tagTitle', () => {
+        expect(
+            getPageFullTitle(makeContext('SEO title'), makePage('Page title', 'SEO title'))
+        ).toBe('SEO title');
+    });
+});

packages/gitbook/src/components/SitePage/SitePage.tsx

@@ -4,7 +4,6 @@ import { notFound, redirect } from 'next/navigation';
 import {
     CustomizationDefaultThemeMode,
     CustomizationHeaderPreset,
-    type RevisionPageDocument,
     SiteInsightsDisplayContext,
     type TranslationLanguage,
 } from '@gitbook/api';
@@ -18,6 +17,7 @@ import {
     getPathnameParam,
 } from './fetch';
 import { PageClientLayout } from './PageClientLayout';
+import { getPageFullTitle } from './title';
 import { UpdatesFilterProvider } from '@/components/DocumentView/UpdatesFilter';
 import { UpdatesFilterScript } from '@/components/DocumentView/UpdatesFilterScript';
 import { PageAside } from '@/components/PageAside';
@@ -33,11 +33,7 @@ import { getResizedImageURL } from '@/lib/images';
 import { getPagePath } from '@/lib/pages';
 import { resolveContentRef } from '@/lib/references';
 import { isPageIndexable, isSiteIndexable } from '@/lib/seo';
-import {
-    getSiteSpacePagePaths,
-    getSiteStructureTitle,
-    resolveSiteSpaceCustomHomePage,
-} from '@/lib/sites';
+import { getSiteSpacePagePaths, resolveSiteSpaceCustomHomePage } from '@/lib/sites';
 import { tcls } from '@/lib/tailwind';
 import {
     generateUpdatesFilterCSS,
@@ -45,6 +41,8 @@ import {
     updatesFilterStyleHref,
 } from '@/lib/updates';
 
+export { getPageFullTitle } from './title';
+
 export type SitePageProps = {
     context: GitBookSiteContext;
     pageParams: PagePathParams;
@@ -421,20 +419,3 @@ async function resolvePageMetaLinks(
         alternates: [],
     };
 }
-
-/**
- * Get the <title> for a page.
- */
-export function getPageFullTitle(context: GitBookSiteContext, page: RevisionPageDocument) {
-    const { site } = context;
-    const siteStructureTitle = getSiteStructureTitle(context);
-
-    return [
-        page.title,
-        // Prevent duplicate titles by comparing against the page title.
-        page.title !== siteStructureTitle ? siteStructureTitle : null, // The first page of a section is often the same as the section title, so we don't need to show it.
-        page.title !== site.title ? site.title : null, // The site title can also be the same as the site title on the site's landing page.
-    ]
-        .filter(Boolean)
-        .join(' | ');
-}

packages/gitbook/src/components/SitePage/title.ts

@@ -0,0 +1,26 @@
+import type { RevisionPageDocument } from '@gitbook/api';
+
+import type { GitBookSiteContext } from '@/lib/context';
+import { getSiteStructureTitle } from '@/lib/sites';
+
+// TODO(RND-11994): drop once @gitbook/api ships RevisionPageDocument.tagTitle
+type PageWithTagTitle = RevisionPageDocument & { tagTitle?: string };
+
+/**
+ * Get the <title> for a page.
+ */
+export function getPageFullTitle(context: GitBookSiteContext, page: RevisionPageDocument) {
+    const { site } = context;
+    const siteStructureTitle = getSiteStructureTitle(context);
+    const tagTitle = (page as PageWithTagTitle).tagTitle || page.title;
+
+    return [
+        tagTitle,
+        // The first page of a section is often the same as the section title, so we don't need to show it.
+        tagTitle !== siteStructureTitle ? siteStructureTitle : null,
+        // The site title can also be the same as the page title on the site's landing page.
+        tagTitle !== site.title ? site.title : null,
+    ]
+        .filter(Boolean)
+        .join(' | ');
+}