fix(ui): remove dead scroll on pages that fit the window

[BigSimmo]

Summary

• Home pages and other pages whose content already ended still carried a scroll range, so a scrollbar appeared on a page with nothing below the fold and a wheel notch jolted the page into its bottom stop. Measured in Chromium across 39 routes at five viewports: 8px on every standalone page, 38–46px on all 15 shared mode homes, 57px on routes carrying the header nav row, and up to 273px on a tall window. The amount did not vary with window size, so it was present on every window, permanently.
• The cause was the same everywhere: page-fill floors written as calc(100dvh - <chrome estimate>), and no estimate could be right. --shell-header-h (4rem) covers the header's inner min-h-14 bar plus its pb-2 but not the bar's own pt-[max(0.5rem,var(--safe-area-top))]; nothing knew about the header-collapse-addon nav row that topic routes mount; nothing knew about #main-content's own sm:pb-8; and the dashboard's 11rem guess had to cover the header block, the wrapper padding, the desktop composer slot and the space-y gap in one hard-coded number.
• Surfaces now grow into the box above them, which is exact by construction and cannot drift again: shell #main-content grows into .phone-viewport-frame (sm:grow); mobile-composer-reserve-pad becomes the fill box at sm+ (sm:flex sm:min-h-full sm:flex-col); page shells grow into that pad (sm:grow); and the dashboard content wrapper is the fill box for the mode-home canvas, which grows into it (sm:grow sm:shrink-0).
• Phone geometry is untouched. Below sm the document owns scrolling and there is no bounded box to fill, so the phone floors stay as they were. All 65 phone measurements are byte-identical before and after.
• Adds the "pages that fit the window have no scroll range" guard to tests/ui-chrome-scroll.spec.ts — the assertion whose absence let this sit unnoticed.
• Records the rule as invariant 24 in docs/search-chrome-behaviour.md and corrects the --shell-header-h comment in globals.css, which had claimed the token existed so the header height and the page-fill floors "cannot silently drift apart".
• Updates the two contract tests that pinned the old floor strings (ui-overlay-css-contract, mobile-interaction-regressions) to pin the new fill contract, including negative assertions so the estimate cannot come back.

Verification

• npm run verify:pr-local

Run locally instead, and reported exactly as run:

• npm run typecheck — clean.
• npm run lint — clean, --max-warnings 0.
• npm run test (full offline unit suite) — 10,814 passed, 4 skipped, 3 failed. The three failures (clinical-hazard-controls, privacy-readiness-contract, rag-plan-package-parity) were confirmed pre-existing: they fail identically with this branch's src/, tests/ and docs/ changes stashed. rag-plan-package-parity fails only because the manifest's reconciled base commit is not present in this shallow clone.
• npm run verify:phone-chrome — contracts stage passed (9 files, 135 tests passed, including header-scroll-hide-contract, use-hide-on-scroll, mobile-composer-reserve and ui-overlay-css-contract). Its browser stages could not run here, see below.
• Browser measurement, Chromium, 39 routes × 5 viewports (390×844 phone, 700×900, 834×1112 tablet, 1440×900, 1280×1200), before and after: 82 measurements improved, 113 unchanged, 0 increased, 0 routes with horizontal overflow. Every page that fits now reports a scroll range of exactly 0; every page with real content reports the same range as before. A separate 1440×2000 pass confirmed the previously hidden cases (/favourites 40→0, /dictionary/topics 273→0, /calculators/search 224→0, /specifiers/map 273→0, /differentials/compare 273→0). Screenshots of a mode home, a standalone info page, a nav-row route, a results view and a phone home were inspected: nothing shifted, centring still reads correctly, and the background still reaches the bottom edge.
• The four assertions the new guard makes were run directly at its exact viewport (1440×1200) against /, /?mode=documents, /medications and /factsheets/topics — all report maxScrollTop = 0 on both the settled and the stability read.

UI verification not run: npm run verify:ui and the verify:phone-chrome browser stages could not execute in this container. The lock pins Playwright chromium-1234; the image ships only chromium-1194, and both remedies are blocked by the environment's proxy — scripts/setup-claude-cloud.sh browsers fails on 403 from the apt PPAs, and npx playwright install chromium into the managed cache fails with a download error. Per the #255 guard in scripts/check-playwright-browser-revision.mjs, pointing PLAYWRIGHT_CHROMIUM_EXECUTABLE_PATH at the mismatched shell is forbidden, so browser proof is delegated to CI Production UI. The browser measurements above were taken with the container's chromium-1194 against the dev server, not the pinned revision against a production build, and are offered as evidence of the geometry change, not as the Chromium gate.

• npm run verify:release before release or handoff confidence claims

Risk and rollout

• Risk: Shared layout foundations. Four surfaces change from a viewport-derived min-height floor to flex growth, and two containers become flex columns at sm+ (mobile-composer-reserve-pad, the dashboard content wrapper). The realistic failure mode is a page that no longer stretches to the bottom — visually neutral where it happens, because the shell and every one of these surfaces paint the same var(--background), but worth a look on wide layouts. Percentage resolution is one level deep in both chains (min-h-full against a parent with a definite height), which is why the nested favourites grid was moved to sm:min-h-full against its now-grown hub rather than left on a viewport calc.
• Rollback: Single commit, revert cleanly. No data, schema, or configuration change.
• Provider or production effects: None. No Supabase, OpenAI, CI, or deployment surface touched; no provider-backed command was run.
• RAG impact: none — no retrieval, ranking, selection, or answer surface is touched.

Notes

• Two residuals were measured and deliberately left, both outside this change's cause:
  • /calculators/search at 1280×1200 keeps a 2px range. That is the catalogue grid's own content being 2px taller than its space, not a chrome estimate; no floor binds there any more.
  • mode-home-page-skeleton.tsx still carries sm:min-h-[calc(100dvh-var(--shell-header-h))]. It is a route loading.tsx placeholder whose parent chain is a Suspense boundary rather than the reserve pad, so growth would not resolve there. It is transient and cannot produce the persistent scrollbar this PR fixes.
• Two console messages seen while testing are pre-existing and unrelated — a getServerSnapshot should be cached warning on /favourites and a 404 for one resource on /. Both reproduce identically with this branch's changes stashed.
• The --shell-header-h token keeps its value and its other consumers (chrome offsets, --phone-overlay-chrome-h, which adds the top pad back explicitly). Only its use as a page-fill floor is retired.

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Generated by Claude Code

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Note

Medium Risk
Touches shared shell flex geometry (#main-content, reserve pad, dashboard wrapper); failure mode is pages not stretching to the bottom on wide layouts, though backgrounds should mask it.

Overview
Fixes phantom scrollbars on desktop/tablet (sm+) when page content already fits the viewport. Floors built as calc(100dvh - <chrome estimate>) systematically overshot because no static number could account for the header’s top safe-area pad, optional header-collapse-addon nav row, or #main-content’s bottom padding — leaving roughly 8–273px of empty scroll range.

Layout contract change: at sm+, #main-content and the mobile-composer-reserve-pad become a definite flex fill chain (sm:grow, sm:flex sm:min-h-full sm:flex-col); page shells, dashboard content wrapper, mode-home canvas, and shared primitives (searchPageShell, InformationPageShell, therapy/differentials/favourites mains) switch from viewport min-h calcs to sm:grow (favourites split rail uses sm:min-h-full against the grown hub). Phone viewport calcs are unchanged where the document still owns scrolling.

Documents invariant 24, clarifies --shell-header-h is for offsets—not page fill—and adds Playwright “pages that fit the window have no scroll range” plus contract-test pins so the old estimate pattern cannot return.

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⛔ Files ignored due to path filters (1)

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📒 Files selected for processing (15)

• src/app/globals.css
• src/components/ClinicalDashboard.tsx
• src/components/clinical-dashboard/favourites-command-library-page.tsx
• src/components/clinical-dashboard/global-search-shell.tsx
• src/components/clinical-dashboard/mode-home-canvas.ts
• src/components/differentials/differential-compare-queue-page.tsx
• src/components/differentials/differential-presentation-workflow-page.tsx
• src/components/differentials/differential-stream-workspace.tsx
• src/components/information-page-shell.tsx
• src/components/mode-home-template.tsx
• src/components/therapy-compass/workspace.tsx
• src/components/ui-primitives.tsx
• tests/mobile-interaction-regressions.test.ts
• tests/ui-chrome-scroll.spec.ts
• tests/ui-overlay-css-contract.test.ts

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