# Phase 12 — One Header, Same Size Everywhere

**Story:** `.agent/user_stories/header-consistency.md`
**Context:** owner report 2026-08-22 — "the header changes size between
sources and chat."

## Goal
The sticky top bar is the **same height on every page**: Chat, Sources,
and the document viewer (the page whose back button says "Sources" — the
one users compare against chat).

## Diagnosis (measured 2026-08-22, headless Chromium)
- Desktop: `.app-header` = 64px on `/` **and** `/sources.html`
  (identical markup/CSS); `.doc-header` = **66.375px** (content-sized:
  `padding-block: 0.7rem` around a 44px back pill + 2-line title block).
- Mobile (≤640px): app header 58px; `.doc-header` = **63.19px** (and
  grows further when the title/meta wrap).
So the "sources header" (viewer back button = "Sources") is visibly
taller than the chat header — on desktop and mobile.

## Implementation steps
1. **CSS** (`frontend/assets/styles.css`):
   - `.doc-header { height: var(--header-h); }` — same fixed box as
     `.app-header`.
   - `.doc-header-inner { height: 100%; padding-block: 0; }` (center
     vertically; the 44px pill + title/meta block fit in 64/58px).
   - Mobile: replace the wrapping rule with `flex-wrap: nowrap`;
     `.doc-meta { flex-wrap: nowrap; overflow: hidden; }` so the meta
     row clips (path already ellipsizes) instead of growing the header.
     `#doc-title` already ellipsizes on one line.
2. No HTML/JS changes (ids/classes unchanged → phase-10 tests intact).
3. **PLAN §7.1:** note the shared 64px header bar across all three pages.

## Locked decisions
None. A11/Phase-08 tokens untouched (same colors, same hairline).

## Testing & Quality
- E2E: `tests/e2e/test_header_consistency.py` per the story mapping
  (desktop 1280px + mobile 375px: three pages, three identical boxes).
- Regression: `test_document_viewer.py` green in isolation (viewer
  content/meta/back link unchanged).
- Coverage gate unchanged (no `app/` code touched → stays >90%).

## Commit
```bash
git add -A .agent/ frontend/assets/styles.css tests/e2e/test_header_consistency.py && git commit --no-gpg-sign -m "fix(ui): uniform header bar height on chat, sources, and the document viewer"
```