## Summary
Mirror of #176 onto `portal-shell`. Same one-line shield, same rationale: the two apps share an identical `:host { height: 100vh }` + `<main> overflow-y: auto` layout, so the same defensive `html, body { overflow-y: hidden }` rule belongs on both surfaces. Brings the public-facing shell to the same posture as the admin one — any future layout escape stops at the shell boundary rather than producing a phantom body scrollbar plus an empty band below the footer.
## What lands
`apps/portal-shell/src/styles.css`:
```css
html,
body {
overflow-y: hidden;
}
```
Same block as #176 with a comment that explicitly cross-references the admin shield so future contributors don't accidentally diverge the two apps.
## Notes for the reviewer
- **Why now, rather than waiting for portal-shell to demonstrate the symptom?** #176's reviewer note said this would land "if/when a layout escape shows up there." The user asked for parity immediately, on the reasoning that the shell contract is the *same* on both apps — the shield is defensive and one-line, so coupling the two posture changes is cheaper than tracking a "TODO once we see it in shell".
- **No new tests.** Same justification as #176 — the change is at the global stylesheet level, has no behavioural surface, and the manual repro path already exists (force a chart or wide element past the viewport; pre-shield → body scrollbar + footer gap; post-shield → clipped at the shell root, `<main>` still scrolls).
- **Element-level scrolling on `<main>` is unaffected.** The skip-link, sidebar, and footer all keep their pinned positions; long routes (the user list, future content pages) scroll inside `<main>` as designed.
## Test plan
- [x] `pnpm nx build portal-shell` — clean.
- [x] `pnpm nx test portal-shell` — green.
- [x] `pnpm nx lint portal-shell` — clean.
- [x] `pnpm nx run-many -t build test lint -p portal-shell,portal-admin` — green (admin and shell share the build cache; touching only one file invalidates only the shell target).
- [ ] **Manual smoke** — `pnpm nx serve portal-shell`:
- Open `/fr` and `/en`, scroll long pages — `<main>` scrolls, body doesn't.
- Resize across the breakpoint where the sidebar collapses — body still doesn't scroll; sidebar/footer pin correctly.
- Toggle dark mode — no visual regression.
## What's next
Nothing pending on this shell-shield front. The two apps are now symmetric; if a third app appears (it won't in v1) the same pattern is documented in both `styles.css` headers.
---------
Co-authored-by: Julien Gautier <julien.gautier@apf.asso.fr>
Reviewed-on: #177
julien
b9daaa5f58