fix: strip build-time basePath from router.push redirects after login #390

lib/__tests__/base-path.test.ts

@@ -72,3 +72,33 @@ describe('withBasePath — asset-URL fallbacks under a subpath', () => {
     expect(getPathPrefix()).toBe('/webmail');
   });
 });
+
+describe('toRouterPath — router.push paths under a subpath', () => {
+  // With a build-time basePath, Next's router prepends the prefix itself, so
+  // browser-derived paths (redirect_after_login stores
+  // window.location.pathname) must be stripped or the redirect lands on
+  // /webmail/webmail/en. See #390.
+  it('strips the static base path from a stored browser path', async () => {
+    const { toRouterPath } = await loadNav('/webmail');
+    expect(toRouterPath('/webmail/en')).toBe('/en');
+    expect(toRouterPath('/webmail/en/calendar?view=day')).toBe('/en/calendar?view=day');
+    expect(toRouterPath('/webmail')).toBe('/');
+    expect(toRouterPath('/webmail?compose=1')).toBe('/?compose=1');
+  });
+
+  it('leaves already-stripped and unrelated paths alone', async () => {
+    const { toRouterPath } = await loadNav('/webmail');
+    expect(toRouterPath('/en')).toBe('/en');
+    expect(toRouterPath('/')).toBe('/');
+    // Shares the prefix text but is a different first segment.
+    expect(toRouterPath('/webmail2/en')).toBe('/webmail2/en');
+  });
+
+  it('passes paths through unchanged when no base path is built in', async () => {
+    // Legacy runtime-detected proxy mounts: Next knows nothing about the
+    // prefix, so router.push needs the full prefixed path.
+    const { toRouterPath } = await loadNav(undefined);
+    expect(toRouterPath('/webmail/en')).toBe('/webmail/en');
+    expect(toRouterPath('/en')).toBe('/en');
+  });
+});

lib/browser-navigation.ts

@@ -96,6 +96,27 @@ export function withBasePath(url: string | null | undefined): string {
 }
 
 
+/**
+ * Converts a browser-style path (as found in `window.location.pathname`,
+ * which always includes the mount prefix) into a path safe to hand to Next's
+ * client router (`router.push` / `router.replace`).
+ *
+ * When the app is built with NEXT_PUBLIC_BASE_PATH, Next's router prepends
+ * the basePath itself, so a stored prefixed path would get it twice (#390) —
+ * strip it here. Legacy runtime-detected proxy mounts pass through unchanged:
+ * Next knows nothing about that prefix, so the router needs the full path.
+ *
+ * Accepts paths with query/hash suffixes (`/webmail/en/calendar?view=day`).
+ */
+export function toRouterPath(path: string): string {
+  if (!STATIC_BASE_PATH || !path.startsWith(STATIC_BASE_PATH)) return path;
+  const rest = path.slice(STATIC_BASE_PATH.length);
+  if (rest === '') return '/';
+  if (rest[0] === '/') return rest;
+  if (rest[0] === '?' || rest[0] === '#') return '/' + rest;
+  return path; // different first segment that merely shares the prefix text
+}
+
 /**
  * Extracts the locale from the current URL, skipping any mount prefix.
  * Falls back to 'en' when no known locale segment is found.