Root cause: `Dashboard.tsx:194` runs its own `!sessionId && !isImpersonating &&
!getDevUser()` auth guard, redirecting to `/login` if `sessionId` is null. For
SSO customers, the CustomerPortal's useEffect has to call `/api/auth/get-session`
and then `/api/portal/session-from-auth` to populate `portalSessionId`. During
that bootstrap window (typically 100-300ms), `sessionId` is null and the guard
fires — redirecting the user to `/login` and breaking the post-sign-in flow.
App.tsx additionally returned `null` at `/login` for authenticated users
(`showCustomerPortal` is false at `/login`), leaving a blank React root even
if the redirect target was /login itself.
Fix:
- `CustomerPortal.tsx`: show a 'Loading…' state (`role=status`) while
`!initComplete`. The portal chrome and its child sections only mount once
the bootstrap has resolved, so child auth guards don't fire prematurely.
- `App.tsx`: at `/login` with a valid session, redirect to `/` so the
customer lands on the portal instead of seeing a blank page.
- `App.tsx`: only return `LoginPage` when at `/login` — other portal
routes defer the auth check to `CustomerPortal` (the customer SSO bridge
resolves `portalSessionId` on mount).
- `UAT_PLAYBOOK.md`: add §5.27 with 8 cases covering the bug, the loading
state, the /login auto-redirect, the unauth fallback, and the groomer /
impersonation non-regressions.
- `src/__tests__/portal.test.tsx`: add a regression test that asserts the
loading state is shown during the bridge and the portal nav is NOT in the
DOM mid-bootstrap.
Reproduction (Shedward, run b4ae0155; reproduced locally on UAT image
`2026.06.01-ec29f71`):
1. From `about:blank`, complete customer SSO as `uat-customer`.
2. `browser_navigate` to `/portal`.
3. Pre-fix: redirected to `/login` with blank React root.
4. Post-fix: URL stays at `/portal`, dashboard renders with customer name.
Co-Authored-By: Paperclip <noreply@paperclip.ing>
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