feat: warn before leaving a form with unsaved changes

Drag-dropping page images then clicking a nav link silently discarded the
whole form. Add a reusable guardUnsavedChanges helper (SvelteKit beforeNavigate
confirm + native beforeunload) and wire it into the manga upload, manga edit,
and chapter upload forms. The dirty check returns false while submitting so the
post-save redirect isn't prompted.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/e2e/unsaved-guard.spec.ts

@@ -0,0 +1,59 @@
+import { test, expect, type Page } from './fixtures';
+
+// Leaving a form with unsaved work (e.g. a half-filled upload) must prompt so a
+// misclicked nav link doesn't silently discard everything.
+
+const DESKTOP = { width: 1280, height: 720 } as const;
+
+async function authed(page: Page) {
+    await page.route('**/api/v1/auth/config', (r) =>
+        r.fulfill({
+            status: 200,
+            contentType: 'application/json',
+            body: JSON.stringify({ self_register_enabled: true, private_mode: false })
+        })
+    );
+    await page.route('**/api/v1/auth/me', (r) =>
+        r.fulfill({
+            status: 200,
+            contentType: 'application/json',
+            body: JSON.stringify({
+                user: { id: 'u1', username: 'uploader', created_at: '2026-01-01T00:00:00Z', is_admin: false }
+            })
+        })
+    );
+    await page.route('**/api/v1/auth/me/preferences', (r) =>
+        r.fulfill({ status: 200, contentType: 'application/json', body: '{}' })
+    );
+    await page.route('**/api/v1/genres', (r) =>
+        r.fulfill({ status: 200, contentType: 'application/json', body: '[]' })
+    );
+    await page.route('**/api/v1/me/bookmarks*', (r) =>
+        r.fulfill({
+            status: 200,
+            contentType: 'application/json',
+            body: JSON.stringify({ items: [], page: { limit: 50, offset: 0, total: 0 } })
+        })
+    );
+}
+
+test('upload form prompts before navigating away with unsaved changes', async ({ page }) => {
+    await authed(page);
+    await page.setViewportSize(DESKTOP);
+    await page.goto('/upload');
+
+    // Make the form dirty.
+    await page.getByLabel(/Title/).fill('WIP Manga');
+
+    // Cancel the confirm → navigation is blocked, we stay put with our input.
+    let dialogSeen = false;
+    page.once('dialog', (d) => {
+        dialogSeen = true;
+        d.dismiss();
+    });
+    await page.getByRole('link', { name: 'Bookmarks' }).click();
+
+    await expect.poll(() => dialogSeen).toBe(true);
+    await expect(page).toHaveURL(/\/upload/);
+    await expect(page.getByLabel(/Title/)).toHaveValue('WIP Manga');
+});

frontend/package.json

@@ -1,6 +1,6 @@
 {
   "name": "mangalord-frontend",
-  "version": "0.127.2",
+  "version": "0.128.0",
   "private": true,
   "type": "module",
   "scripts": {

frontend/src/lib/unsaved-guard.ts

@@ -0,0 +1,36 @@
+import { beforeNavigate } from '$app/navigation';
+import { onMount } from 'svelte';
+
+/**
+ * Warn before leaving a page with unsaved form state. Call once from a
+ * component's script (during init). `isDirty` is read live on each navigation,
+ * so pass a closure over the form's reactive state — and have it return `false`
+ * once a save has succeeded (or is in flight) so the post-save redirect doesn't
+ * prompt.
+ *
+ * Covers both in-app navigation (SvelteKit `beforeNavigate`, with a confirm) and
+ * full-page unload / tab close / reload (the native `beforeunload` prompt).
+ */
+export function guardUnsavedChanges(isDirty: () => boolean): void {
+    beforeNavigate((nav) => {
+        // `leave` (tab close / reload) can't show a custom confirm here — the
+        // beforeunload handler below covers it. Only guard in-app navigations.
+        if (nav.type === 'leave') return;
+        if (!isDirty()) return;
+        if (!confirm('You have unsaved changes. Leave this page and discard them?')) {
+            nav.cancel();
+        }
+    });
+
+    onMount(() => {
+        const handler = (e: BeforeUnloadEvent) => {
+            if (isDirty()) {
+                e.preventDefault();
+                // Legacy browsers require returnValue to be set.
+                e.returnValue = '';
+            }
+        };
+        window.addEventListener('beforeunload', handler);
+        return () => window.removeEventListener('beforeunload', handler);
+    });
+}

frontend/src/routes/manga/[id]/edit/+page.svelte

@@ -1,6 +1,7 @@
 <script lang="ts">
     import { goto } from '$app/navigation';
     import { ApiError, fileUrl } from '$lib/api/client';
+    import { guardUnsavedChanges } from '$lib/unsaved-guard';
     import {
         deleteMangaCover,
         updateManga,
@@ -49,6 +50,25 @@
         mangaTitle.trim().length > 0 && !coverError && !submitting
     );
 
+    // Warn before leaving with unsaved edits. `submitting` stays true through
+    // the post-save redirect, so that navigation isn't prompted.
+    guardUnsavedChanges(
+        () =>
+            !submitting &&
+            (mangaTitle !== data.manga.title ||
+                mangaStatus !== data.manga.status ||
+                mangaDescription !== (data.manga.description ?? '') ||
+                JSON.stringify(mangaAuthors) !==
+                    JSON.stringify(data.manga.authors.map((a) => a.name)) ||
+                JSON.stringify(mangaAltTitles) !== JSON.stringify([...data.manga.alt_titles]) ||
+                JSON.stringify(mangaGenreIds) !==
+                    JSON.stringify(data.manga.genres.map((g) => g.id)) ||
+                coverFile != null ||
+                pendingCoverRemoval ||
+                authorDraft.trim() !== '' ||
+                altTitleDraft.trim() !== '')
+    );
+
     function addAuthor() {
         const name = authorDraft.trim();
         if (!name) return;

frontend/src/routes/manga/[id]/upload-chapter/+page.svelte

@@ -1,6 +1,7 @@
 <script lang="ts">
     import { goto } from '$app/navigation';
     import { ApiError, fileUrl } from '$lib/api/client';
+    import { guardUnsavedChanges } from '$lib/unsaved-guard';
     import { createChapter } from '$lib/api/chapters';
     import { session } from '$lib/session.svelte';
     import ChapterPagesEditor, {
@@ -19,6 +20,16 @@
     let submitting = $state(false);
     let error: string | null = $state(null);
 
+    // Warn before leaving with staged pages / edits. `submitting` stays true
+    // through the post-save redirect, so that navigation isn't prompted.
+    guardUnsavedChanges(
+        () =>
+            !submitting &&
+            (pages.length > 0 ||
+                title.trim() !== '' ||
+                (number != null && number !== data.defaultNumber))
+    );
+
     const allPagesValid = $derived(pages.every((p) => !p.error));
     const canSubmit = $derived(
         Boolean(session.user) &&

frontend/src/routes/upload/+page.svelte

@@ -1,6 +1,7 @@
 <script lang="ts">
     import { goto } from '$app/navigation';
     import { ApiError } from '$lib/api/client';
+    import { guardUnsavedChanges } from '$lib/unsaved-guard';
     import { createManga, type MangaStatus } from '$lib/api/mangas';
     import { createChapter } from '$lib/api/chapters';
     import { session } from '$lib/session.svelte';
@@ -43,6 +44,23 @@
     let mangaError = $state<string | null>(null);
     let success = $state<string | null>(null);
 
+    // Warn before navigating away with an in-progress upload. Suppressed while
+    // submitting and after a successful save (which redirects).
+    guardUnsavedChanges(
+        () =>
+            !success &&
+            !submitting &&
+            (mangaTitle.trim() !== '' ||
+                mangaDescription.trim() !== '' ||
+                mangaAuthors.length > 0 ||
+                mangaAltTitles.length > 0 ||
+                mangaGenreIds.length > 0 ||
+                coverFile != null ||
+                stagedChapters.length > 0 ||
+                authorDraft.trim() !== '' ||
+                altTitleDraft.trim() !== '')
+    );
+
     const allChapterPagesValid = $derived(
         stagedChapters.every((c) => c.pages.every((p) => !p.error))
     );