sync_ui_cookie() was calling window.parent.location.reload() on every
render when user.yaml has ui_preference=vue, but no Caddy is in the
traffic path (test instances, bare Docker). This caused an infinite
reload loop because the reload just came back to Streamlit.
Gate the reload on PEREGRINE_CADDY_PROXY=1. Without it, the cookie is
still written silently but no reload is attempted. Add the env var to
compose.yml and compose.cloud.yml (both are behind Caddy); omit from
compose.test-cfcore.yml so test instances stay stable.
- ui_switcher.py: add explicit guard that forces pref=streamlit when
DEMO_MODE=true, before the tier-downgrade check. Demo Vue SPA (#46)
is not yet implemented, so navigating there produced a blank screen.
- app.py: call sync_ui_cookie inside wizard gate block before st.stop()
so that cloud users with ui_preference=vue are redirected correctly
even when the first-run wizard is still active. Previous behaviour
called sync_ui_cookie after pg.run() which was never reached.
- demo/config/user.yaml: reset ui_preference to streamlit (belt-and-
suspenders alongside the code guard).
Closes: demo blank-screen regression reported 2026-03-24.
- Add explanatory comments to all 5 bare except Exception blocks clarifying that UI components must not crash the app
- Refactor sync_ui_cookie() to load UserProfile once instead of up to 3 times in normal path
- Store profile reference and reuse it in tier downgrade protection block
- Replace importlib.reload() pattern in tests with unittest.mock.patch for _DEMO_MODE
- Improves test isolation and eliminates module state contamination across test runs
- All 5 tests pass (100%)
